Thanks. But did not work. I tried this before (really tried everything I could imagine).

.... whereas the list of duplicate bugs in bug #1571865 rises and rises ;-(  But I am shure guys at ubuntu will fix this.

Serious admins would not update critical systems for now. Mine is only the entertainment server and I can live a few days without mythtv ;-)

Just for note:

#> systemctl enable mysql.service
#> service mysql start
#> tail -f /var/log/mysql/error.log

2016-04-26T15:35:03.306607Z 0 [Warning] Changed limits: max_open_files: 1024 (requested 5000)
2016-04-26T15:35:03.306733Z 0 [Warning] Changed limits: table_open_cache: 457 (requested 2000)
2016-04-26T15:35:03.550179Z 0 [Warning] option 'net_buffer_length': unsigned value 8388608 adjusted to 1048576
2016-04-26T15:35:03.550287Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2016-04-26T15:35:03.551657Z 0 [Note] /usr/sbin/mysqld (mysqld 5.7.12-0ubuntu1) starting as process 12296 ...
2016-04-26T15:35:03.561978Z 0 [Note] InnoDB: PUNCH HOLE support available
2016-04-26T15:35:03.562120Z 0 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2016-04-26T15:35:03.562133Z 0 [Note] InnoDB: Uses event mutexes
2016-04-26T15:35:03.562143Z 0 [Note] InnoDB: GCC builtin __atomic_thread_fence() is used for memory barrier
2016-04-26T15:35:03.562153Z 0 [Note] InnoDB: Compressed tables use zlib 1.2.8
2016-04-26T15:35:03.562168Z 0 [Note] InnoDB: Using Linux native AIO
2016-04-26T15:35:03.562712Z 0 [Note] InnoDB: Number of pools: 1
2016-04-26T15:35:03.562985Z 0 [Note] InnoDB: Not using CPU crc32 instructions
2016-04-26T15:35:03.584765Z 0 [Note] InnoDB: Initializing buffer pool, total size = 128M, instances = 1, chunk size = 128M
2016-04-26T15:35:03.608347Z 0 [Note] InnoDB: Completed initialization of buffer pool
2016-04-26T15:35:03.613754Z 0 [Note] InnoDB: If the mysqld execution user is authorized, page cleaner thread priority can be changed. See the man page of setpriority().
2016-04-26T15:35:03.629460Z 0 [Note] InnoDB: Highest supported file format is Barracuda.
2016-04-26T15:35:03.680261Z 0 [Note] InnoDB: Creating shared tablespace for temporary tables
2016-04-26T15:35:03.680485Z 0 [Note] InnoDB: Setting file './ibtmp1' size to 12 MB. Physically writing the file full; Please wait ...
2016-04-26T15:35:03.778536Z 0 [Note] InnoDB: File './ibtmp1' size is now 12 MB.
2016-04-26T15:35:03.781534Z 0 [Note] InnoDB: 96 redo rollback segment(s) found. 96 redo rollback segment(s) are active.
2016-04-26T15:35:03.781618Z 0 [Note] InnoDB: 32 non-redo rollback segment(s) are active.
2016-04-26T15:35:03.783864Z 0 [Note] InnoDB: Waiting for purge to start
2016-04-26T15:35:03.834409Z 0 [Note] InnoDB: 5.7.12 started; log sequence number 166629941
2016-04-26T15:35:03.834940Z 0 [Note] InnoDB: Loading buffer pool(s) from /var/lib/mysql/ib_buffer_pool
2016-04-26T15:35:03.835109Z 0 [Note] Plugin 'FEDERATED' is disabled.
2016-04-26T15:35:03.835306Z 0 [Note] InnoDB: Buffer pool(s) load completed at 160426 17:35:03
2016-04-26T15:35:03.836621Z 0 [Warning] System table 'plugin' is expected to be transactional.
2016-04-26T15:35:03.837061Z 0 [ERROR] unknown variable 'table_cache=128'
2016-04-26T15:35:03.837080Z 0 [ERROR] Aborting

2016-04-26T15:35:03.837093Z 0 [Note] Binlog end
2016-04-26T15:35:03.837183Z 0 [Note] Shutting down plugin 'ngram'
2016-04-26T15:35:03.837195Z 0 [Note] Shutting down plugin 'ARCHIVE'
2016-04-26T15:35:03.837202Z 0 [Note] Shutting down plugin 'partition'
2016-04-26T15:35:03.837209Z 0 [Note] Shutting down plugin 'BLACKHOLE'
2016-04-26T15:35:03.837215Z 0 [Note] Shutting down plugin 'INNODB_SYS_VIRTUAL'
2016-04-26T15:35:03.837223Z 0 [Note] Shutting down plugin 'INNODB_SYS_DATAFILES'
2016-04-26T15:35:03.837230Z 0 [Note] Shutting down plugin 'INNODB_SYS_TABLESPACES'
2016-04-26T15:35:03.837236Z 0 [Note] Shutting down plugin 'INNODB_SYS_FOREIGN_COLS'
2016-04-26T15:35:03.837243Z 0 [Note] Shutting down plugin 'INNODB_SYS_FOREIGN'
2016-04-26T15:35:03.837249Z 0 [Note] Shutting down plugin 'INNODB_SYS_FIELDS'
2016-04-26T15:35:03.837255Z 0 [Note] Shutting down plugin 'INNODB_SYS_COLUMNS'
2016-04-26T15:35:03.837262Z 0 [Note] Shutting down plugin 'INNODB_SYS_INDEXES'
2016-04-26T15:35:03.837268Z 0 [Note] Shutting down plugin 'INNODB_SYS_TABLESTATS'
2016-04-26T15:35:03.837275Z 0 [Note] Shutting down plugin 'INNODB_SYS_TABLES'
2016-04-26T15:35:03.837281Z 0 [Note] Shutting down plugin 'INNODB_FT_INDEX_TABLE'
2016-04-26T15:35:03.837287Z 0 [Note] Shutting down plugin 'INNODB_FT_INDEX_CACHE'
2016-04-26T15:35:03.837294Z 0 [Note] Shutting down plugin 'INNODB_FT_CONFIG'
2016-04-26T15:35:03.837300Z 0 [Note] Shutting down plugin 'INNODB_FT_BEING_DELETED'
2016-04-26T15:35:03.837306Z 0 [Note] Shutting down plugin 'INNODB_FT_DELETED'
2016-04-26T15:35:03.837313Z 0 [Note] Shutting down plugin 'INNODB_FT_DEFAULT_STOPWORD'
2016-04-26T15:35:03.837319Z 0 [Note] Shutting down plugin 'INNODB_METRICS'
2016-04-26T15:35:03.837325Z 0 [Note] Shutting down plugin 'INNODB_TEMP_TABLE_INFO'
2016-04-26T15:35:03.837332Z 0 [Note] Shutting down plugin 'INNODB_BUFFER_POOL_STATS'
2016-04-26T15:35:03.837361Z 0 [Note] Shutting down plugin 'INNODB_BUFFER_PAGE_LRU'
2016-04-26T15:35:03.837368Z 0 [Note] Shutting down plugin 'INNODB_BUFFER_PAGE'
2016-04-26T15:35:03.837375Z 0 [Note] Shutting down plugin 'INNODB_CMP_PER_INDEX_RESET'
2016-04-26T15:35:03.837381Z 0 [Note] Shutting down plugin 'INNODB_CMP_PER_INDEX'
2016-04-26T15:35:03.837388Z 0 [Note] Shutting down plugin 'INNODB_CMPMEM_RESET'
2016-04-26T15:35:03.837394Z 0 [Note] Shutting down plugin 'INNODB_CMPMEM'
2016-04-26T15:35:03.837400Z 0 [Note] Shutting down plugin 'INNODB_CMP_RESET'
2016-04-26T15:35:03.837406Z 0 [Note] Shutting down plugin 'INNODB_CMP'
2016-04-26T15:35:03.837413Z 0 [Note] Shutting down plugin 'INNODB_LOCK_WAITS'
2016-04-26T15:35:03.837419Z 0 [Note] Shutting down plugin 'INNODB_LOCKS'
2016-04-26T15:35:03.837425Z 0 [Note] Shutting down plugin 'INNODB_TRX'
2016-04-26T15:35:03.837432Z 0 [Note] Shutting down plugin 'InnoDB'
2016-04-26T15:35:03.837559Z 0 [Note] InnoDB: FTS optimize thread exiting.
2016-04-26T15:35:03.837786Z 0 [Note] InnoDB: Starting shutdown...
2016-04-26T15:35:03.938198Z 0 [Note] InnoDB: Dumping buffer pool(s) to /var/lib/mysql/ib_buffer_pool
2016-04-26T15:35:03.938569Z 0 [Note] InnoDB: Buffer pool(s) dump completed at 160426 17:35:03
2016-04-26T15:35:05.484724Z 0 [Note] InnoDB: Shutdown completed; log sequence number 166629960
2016-04-26T15:35:05.499629Z 0 [Note] InnoDB: Removed temporary tablespace data file: "ibtmp1"
2016-04-26T15:35:05.499693Z 0 [Note] Shutting down plugin 'MEMORY'
2016-04-26T15:35:05.499705Z 0 [Note] Shutting down plugin 'PERFORMANCE_SCHEMA'
2016-04-26T15:35:05.499877Z 0 [Note] Shutting down plugin 'MRG_MYISAM'
2016-04-26T15:35:05.499887Z 0 [Note] Shutting down plugin 'MyISAM'
2016-04-26T15:35:05.499899Z 0 [Note] Shutting down plugin 'CSV'
2016-04-26T15:35:05.499909Z 0 [Note] Shutting down plugin 'sha256_password'
2016-04-26T15:35:05.499917Z 0 [Note] Shutting down plugin 'mysql_native_password'
2016-04-26T15:35:05.500142Z 0 [Note] Shutting down plugin 'binlog'
2016-04-26T15:35:05.501017Z 0 [Note] /usr/sbin/mysqld: Shutdown complete

#> apt-get install mysql-server-5.7
Setting up mysql-server-5.7 (5.7.12-0ubuntu1) ...
initctl: Unable to connect to Upstart: Failed to connect to socket /com/ubuntu/upstart: Connection refused
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
Errors were encountered while processing:
 mysql-server-5.7
E: Sub-process /usr/bin/dpkg returned an error code (1)