Comment 1 for bug 1527535

This is because of when you are going to install percona-server-server-5.7 it will remove percona-server-tokudb-5.6 but not install new percona-server-tokudb-5.7 package :

root@percona-XPS-15:~# sudo apt-get install percona-server-server-5.7
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
  percona-server-client-5.7 percona-server-common-5.7
The following packages will be REMOVED:
  percona-server-client-5.6 percona-server-server-5.6
  percona-server-tokudb-5.6
The following NEW packages will be installed:
  percona-server-client-5.7 percona-server-common-5.7
  percona-server-server-5.7

Process Will end up:

No directory, logging in with HOME=/
.151228 10:46:59 mysqld_safe mysqld_safe must be run as root for setting transparent huge pages!
.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Killing the process:
............../var/lib/dpkg/info/percona-server-server-5.7.postinst: line 87: 5963 Killed invoke-rc.d mysql start
dpkg: error processing package percona-server-server-5.7 (--configure):
 subprocess installed post-installation script returned error exit status 137
Processing triggers for ureadahead (0.100.0-16) ...
Errors were encountered while processing:
 percona-server-server-5.7

If you remove thp-setting=never from my.cnf still will not be able to start PS with TokuDB:

2015-12-28T06:57:08.671881Z 0 [ERROR] Can't open shared library '/usr/lib/mysql/plugin/tokudb_backup.so' (errno: 0 /usr/lib/mysql/plugin/tokudb_backup.so: cannot open shared ob
ject file: No such file or directory)
2015-12-28T06:57:08.671887Z 0 [Warning] Couldn't load plugin named 'tokudb_backup' with soname 'tokudb_backup.so'.
2015-12-28T06:57:08.671901Z 0 [ERROR] Can't open shared library '/usr/lib/mysql/plugin/ha_tokudb.so' (errno: 0 /usr/lib/mysql/plugin/ha_tokudb.so: cannot open shared object fil
e: No such file or directory)
2015-12-28T06:57:08.671905Z 0 [Warning] Couldn't load plugin named 'TokuDB' with soname 'ha_tokudb.so'.
2015-12-28T06:57:08.671914Z 0 [ERROR] Can't open shared library '/usr/lib/mysql/plugin/ha_tokudb.so' (errno: 0 /usr/lib/mysql/plugin/ha_tokudb.so: cannot open shared object fil
e: No such file or directory)
2015-12-28T06:57:08.671917Z 0 [Warning] Couldn't load plugin named 'TokuDB_file_map' with soname 'ha_tokudb.so'.
2015-12-28T06:57:08.671926Z 0 [ERROR] Can't open shared library '/usr/lib/mysql/plugin/ha_tokudb.so' (errno: 0 /usr/lib/mysql/plugin/ha_tokudb.so: cannot open shared object fil
e: No such file or directory)
2015-12-28T06:57:08.671929Z 0 [Warning] Couldn't load plugin named 'TokuDB_fractal_tree_info' with soname 'ha_tokudb.so'.
2015-12-28T06:57:08.671938Z 0 [ERROR] Can't open shared library '/usr/lib/mysql/plugin/ha_tokudb.so' (errno: 0 /usr/lib/mysql/plugin/ha_tokudb.so: cannot open shared object fil
e: No such file or directory)
2015-12-28T06:57:08.671941Z 0 [Warning] Couldn't load plugin named 'TokuDB_fractal_tree_block_map' with soname 'ha_tokudb.so'.
2015-12-28T06:57:08.671949Z 0 [ERROR] Can't open shared library '/usr/lib/mysql/plugin/ha_tokudb.so' (errno: 0 /usr/lib/mysql/plugin/ha_tokudb.so: cannot open shared object fil
e: No such file or directory)
2015-12-28T06:57:08.671952Z 0 [Warning] Couldn't load plugin named 'TokuDB_trx' with soname 'ha_tokudb.so'.
2015-12-28T06:57:08.671961Z 0 [ERROR] Can't open shared library '/usr/lib/mysql/plugin/ha_tokudb.so' (errno: 0 /usr/lib/mysql/plugin/ha_tokudb.so: cannot open shared object fil
e: No such file or directory)
2015-12-28T06:57:08.671969Z 0 [Warning] Couldn't load plugin named 'TokuDB_locks' with soname 'ha_tokudb.so'.

Manually installing percona-server-tokudb-5.7 will resolve this issue:

root@percona-XPS-15:~# sudo apt-get install percona-server-tokudb-5.7
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
  percona-server-tokudb-5.7

mysql> select @@version;
+-----------------+
| @@version |
+-----------------+
| 5.7.10-1rc1-log |
+-----------------+
1 row in set (0.00 sec)

mysql> select plugin_name, plugin_status from information_schema.plugins where plugin_name like '%toku%';
+-------------------------------+---------------+
| plugin_name | plugin_status |
+-------------------------------+---------------+
| TokuDB | ACTIVE |
| TokuDB_file_map | ACTIVE |
| TokuDB_fractal_tree_info | ACTIVE |
| TokuDB_fractal_tree_block_map | ACTIVE |
| TokuDB_trx | ACTIVE |
| TokuDB_locks | ACTIVE |
| TokuDB_lock_waits | ACTIVE |
+-------------------------------+---------------+
7 rows in set (0.00 sec)