mysql failed to upgrade

:~$ sudo apt-get update && sudo apt-get upgrade -y

Setting up mysql-server-5.7 (5.7.12-0ubuntu1) ...
Job for mysql.service failed because the control process exited with error code. See "systemctl status mysql.service" and "journalctl -xe" for details.
invoke-rc.d: initscript mysql, action "start" failed.
dpkg: error processing package mysql-server-5.7 (--configure):
 subprocess installed post-installation script returned error exit status 1
dpkg: dependency problems prevent configuration of mysql-server:
 mysql-server depends on mysql-server-5.7; however:
  Package mysql-server-5.7 is not configured yet.

dpkg: error processing package mysql-server (--configure):
 dependency problems - leaving unconfigured

Errors were encountered while processing:
 mysql-server-5.7
 mysql-server
E: Sub-process /usr/bin/dpkg returned an error code (1)

:~$ journalctl -xe
-- Subject: Unit mysql.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mysql.service has failed.
-- 
-- The result is failed.
May 03 10:50:07 <hostname> systemd[1]: mysql.service: Unit entered failed state.
May 03 10:50:07 <hostname> systemd[1]: mysql.service: Failed with result 'exit-code'.
May 03 10:50:07 <hostname> systemd[1]: mysql.service: Service hold-off time over, scheduling restart.
May 03 10:50:07 <hostname> systemd[1]: Stopped MySQL Community Server.
-- Subject: Unit mysql.service has finished shutting down
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mysql.service has finished shutting down.
May 03 10:50:07 <hostname> systemd[1]: Starting MySQL Community Server...
-- Subject: Unit mysql.service has begun start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mysql.service has begun starting up.
May 03 10:50:09 <hostname> systemd[1]: mysql.service: Main process exited, code=exited, status=1/FAILURE


:~$ vim /var/log/mysql/mysql.log
2016-05-03T00:52:39.082298Z 0 [Warning] Changed limits: max_open_files: 1024 (requested 5000)
2016-05-03T00:52:39.082361Z 0 [Warning] Changed limits: table_open_cache: 431 (requested 2000)
2016-05-03T00:52:39.218362Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2016-05-03T00:52:39.218701Z 0 [Note] /usr/sbin/mysqld (mysqld 5.7.12-0ubuntu1) starting as process 8583 ...
2016-05-03T00:52:39.221607Z 0 [Note] InnoDB: PUNCH HOLE support available
2016-05-03T00:52:39.221626Z 0 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2016-05-03T00:52:39.221631Z 0 [Note] InnoDB: Uses event mutexes
2016-05-03T00:52:39.221638Z 0 [Note] InnoDB: GCC builtin __atomic_thread_fence() is used for memory barrier
2016-05-03T00:52:39.221643Z 0 [Note] InnoDB: Compressed tables use zlib 1.2.8
2016-05-03T00:52:39.221647Z 0 [Note] InnoDB: Using Linux native AIO
2016-05-03T00:52:39.221816Z 0 [Note] InnoDB: Number of pools: 1
2016-05-03T00:52:39.221891Z 0 [Note] InnoDB: Using CPU crc32 instructions
2016-05-03T00:52:39.227090Z 0 [Note] InnoDB: Initializing buffer pool, total size = 128M, instances = 1, chunk size = 128M
2016-05-03T00:52:39.232621Z 0 [Note] InnoDB: Completed initialization of buffer pool
2016-05-03T00:52:39.233838Z 0 [Note] InnoDB: If the mysqld execution user is authorized, page cleaner thread priority can be changed. See the man page of setpriority().
2016-05-03T00:52:39.244953Z 0 [Note] InnoDB: Highest supported file format is Barracuda.
2016-05-03T00:52:39.325891Z 0 [Note] InnoDB: Creating shared tablespace for temporary tables
2016-05-03T00:52:39.325971Z 0 [Note] InnoDB: Setting file './ibtmp1' size to 12 MB. Physically writing the file full; Please wait ...
2016-05-03T00:52:39.492845Z 0 [Note] InnoDB: File './ibtmp1' size is now 12 MB.
2016-05-03T00:52:39.494171Z 0 [Note] InnoDB: 96 redo rollback segment(s) found. 96 redo rollback segment(s) are active.
2016-05-03T00:52:39.494190Z 0 [Note] InnoDB: 32 non-redo rollback segment(s) are active.
2016-05-03T00:52:39.494770Z 0 [Note] InnoDB: Waiting for purge to start
2016-05-03T00:52:39.544928Z 0 [Note] InnoDB: 5.7.12 started; log sequence number 42709746625
2016-05-03T00:52:39.545300Z 0 [Note] InnoDB: Loading buffer pool(s) from /var/lib/mysql/ib_buffer_pool
2016-05-03T00:52:39.545396Z 0 [Note] Plugin 'FEDERATED' is disabled.
2016-05-03T00:52:39.545523Z 0 [Note] InnoDB: Buffer pool(s) load completed at 160503 10:52:39
2016-05-03T00:52:39.545852Z 0 [Warning] System table 'plugin' is expected to be transactional.
2016-05-03T00:52:39.545887Z 0 [ERROR] Function 'innodb' already exists
2016-05-03T00:52:39.545894Z 0 [Warning] Couldn't load plugin named 'innodb' with soname 'ha_innodb.so'.
2016-05-03T00:52:39.545901Z 0 [ERROR] Function 'federated' already exists
2016-05-03T00:52:39.545906Z 0 [Warning] Couldn't load plugin named 'federated' with soname 'ha_federated.so'.
2016-05-03T00:52:39.545911Z 0 [ERROR] Function 'blackhole' already exists
2016-05-03T00:52:39.545916Z 0 [Warning] Couldn't load plugin named 'blackhole' with soname 'ha_blackhole.so'.
2016-05-03T00:52:39.545922Z 0 [ERROR] Function 'archive' already exists
2016-05-03T00:52:39.545926Z 0 [Warning] Couldn't load plugin named 'archive' with soname 'ha_archive.so'.
2016-05-03T00:52:39.546070Z 0 [ERROR] unknown variable 'key_buffer=16M'
2016-05-03T00:52:39.546080Z 0 [ERROR] Aborting

2016-05-03T00:52:39.546089Z 0 [Note] Binlog end
2016-05-03T00:52:39.546129Z 0 [Note] Shutting down plugin 'ngram'
2016-05-03T00:52:39.546137Z 0 [Note] Shutting down plugin 'ARCHIVE'
2016-05-03T00:52:39.546142Z 0 [Note] Shutting down plugin 'partition'
2016-05-03T00:52:39.546146Z 0 [Note] Shutting down plugin 'BLACKHOLE'
2016-05-03T00:52:39.546151Z 0 [Note] Shutting down plugin 'INNODB_SYS_VIRTUAL'
2016-05-03T00:52:39.546156Z 0 [Note] Shutting down plugin 'INNODB_SYS_DATAFILES'
2016-05-03T00:52:39.546161Z 0 [Note] Shutting down plugin 'INNODB_SYS_TABLESPACES'
2016-05-03T00:52:39.546165Z 0 [Note] Shutting down plugin 'INNODB_SYS_FOREIGN_COLS'
2016-05-03T00:52:39.546169Z 0 [Note] Shutting down plugin 'INNODB_SYS_FOREIGN'
2016-05-03T00:52:39.546173Z 0 [Note] Shutting down plugin 'INNODB_SYS_FIELDS'
2016-05-03T00:52:39.546188Z 0 [Note] Shutting down plugin 'INNODB_SYS_COLUMNS'
2016-05-03T00:52:39.546193Z 0 [Note] Shutting down plugin 'INNODB_SYS_INDEXES'
2016-05-03T00:52:39.546197Z 0 [Note] Shutting down plugin 'INNODB_SYS_TABLESTATS'
2016-05-03T00:52:39.546201Z 0 [Note] Shutting down plugin 'INNODB_SYS_TABLES'
2016-05-03T00:52:39.546206Z 0 [Note] Shutting down plugin 'INNODB_FT_INDEX_TABLE'
2016-05-03T00:52:39.546210Z 0 [Note] Shutting down plugin 'INNODB_FT_INDEX_CACHE'
2016-05-03T00:52:39.546214Z 0 [Note] Shutting down plugin 'INNODB_FT_CONFIG'
2016-05-03T00:52:39.546219Z 0 [Note] Shutting down plugin 'INNODB_FT_BEING_DELETED'
2016-05-03T00:52:39.546223Z 0 [Note] Shutting down plugin 'INNODB_FT_DELETED'
2016-05-03T00:52:39.546227Z 0 [Note] Shutting down plugin 'INNODB_FT_DEFAULT_STOPWORD'
2016-05-03T00:52:39.546231Z 0 [Note] Shutting down plugin 'INNODB_METRICS'
2016-05-03T00:52:39.546236Z 0 [Note] Shutting down plugin 'INNODB_TEMP_TABLE_INFO'
2016-05-03T00:52:39.546240Z 0 [Note] Shutting down plugin 'INNODB_BUFFER_POOL_STATS'
2016-05-03T00:52:39.546244Z 0 [Note] Shutting down plugin 'INNODB_BUFFER_PAGE_LRU'
2016-05-03T00:52:39.546248Z 0 [Note] Shutting down plugin 'INNODB_BUFFER_PAGE'
2016-05-03T00:52:39.546252Z 0 [Note] Shutting down plugin 'INNODB_CMP_PER_INDEX_RESET'
2016-05-03T00:52:39.546256Z 0 [Note] Shutting down plugin 'INNODB_CMP_PER_INDEX'
2016-05-03T00:52:39.546261Z 0 [Note] Shutting down plugin 'INNODB_CMPMEM_RESET'
2016-05-03T00:52:39.546265Z 0 [Note] Shutting down plugin 'INNODB_CMPMEM'
2016-05-03T00:52:39.546269Z 0 [Note] Shutting down plugin 'INNODB_CMP_RESET'
2016-05-03T00:52:39.546273Z 0 [Note] Shutting down plugin 'INNODB_CMP'
2016-05-03T00:52:39.546277Z 0 [Note] Shutting down plugin 'INNODB_LOCK_WAITS'
2016-05-03T00:52:39.546281Z 0 [Note] Shutting down plugin 'INNODB_LOCKS'
2016-05-03T00:52:39.546285Z 0 [Note] Shutting down plugin 'INNODB_TRX'
2016-05-03T00:52:39.546289Z 0 [Note] Shutting down plugin 'InnoDB'
2016-05-03T00:52:39.546356Z 0 [Note] InnoDB: FTS optimize thread exiting.
2016-05-03T00:52:39.546518Z 0 [Note] InnoDB: Starting shutdown...
2016-05-03T00:52:39.646698Z 0 [Note] InnoDB: Dumping buffer pool(s) to /var/lib/mysql/ib_buffer_pool
2016-05-03T00:52:39.646904Z 0 [Note] InnoDB: Buffer pool(s) dump completed at 160503 10:52:39
2016-05-03T00:52:41.496639Z 0 [Note] InnoDB: Shutdown completed; log sequence number 42709746644
2016-05-03T00:52:41.499276Z 0 [Note] InnoDB: Removed temporary tablespace data file: "ibtmp1"
2016-05-03T00:52:41.499295Z 0 [Note] Shutting down plugin 'MEMORY'
2016-05-03T00:52:41.499303Z 0 [Note] Shutting down plugin 'PERFORMANCE_SCHEMA'
2016-05-03T00:52:41.499364Z 0 [Note] Shutting down plugin 'MRG_MYISAM'
2016-05-03T00:52:41.499371Z 0 [Note] Shutting down plugin 'MyISAM'
2016-05-03T00:52:41.499378Z 0 [Note] Shutting down plugin 'CSV'
2016-05-03T00:52:41.499385Z 0 [Note] Shutting down plugin 'sha256_password'
2016-05-03T00:52:41.499390Z 0 [Note] Shutting down plugin 'mysql_native_password'
2016-05-03T00:52:41.499575Z 0 [Note] Shutting down plugin 'binlog'
2016-05-03T00:52:41.499757Z 0 [Note] /usr/sbin/mysqld: Shutdown complete