Comment 10 for bug 1562362

I figured out the source of the problem in my case.
It was a memory issue (/var/log/mysql/error.log):

2016-05-20T21:10:32.004395Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2016-05-20T21:10:32.008319Z 0 [Note] /usr/sbin/mysqld (mysqld 5.7.12-0ubuntu1) starting as process 1968 ...
2016-05-20T21:10:32.048643Z 0 [Note] InnoDB: PUNCH HOLE support available
2016-05-20T21:10:32.048704Z 0 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2016-05-20T21:10:32.048719Z 0 [Note] InnoDB: Uses event mutexes
2016-05-20T21:10:32.048730Z 0 [Note] InnoDB: GCC builtin __atomic_thread_fence() is used for memory barrier
2016-05-20T21:10:32.048742Z 0 [Note] InnoDB: Compressed tables use zlib 1.2.8
2016-05-20T21:10:32.048753Z 0 [Note] InnoDB: Using Linux native AIO
2016-05-20T21:10:32.051586Z 0 [Note] InnoDB: Number of pools: 1
2016-05-20T21:10:32.052497Z 0 [Note] InnoDB: Using CPU crc32 instructions
2016-05-20T21:10:32.101825Z 0 [Note] InnoDB: Initializing buffer pool, total size = 128M, instances = 1, chunk size = 128M
2016-05-20T21:10:32.104758Z 0 [ERROR] InnoDB: mmap(137428992 bytes) failed; errno 12
2016-05-20T21:10:32.104802Z 0 [ERROR] InnoDB: Cannot allocate memory for the buffer pool
2016-05-20T21:10:32.105114Z 0 [ERROR] InnoDB: Plugin initialization aborted with error Generic error
2016-05-20T21:10:32.105152Z 0 [ERROR] Plugin 'InnoDB' init function returned error.
2016-05-20T21:10:32.105175Z 0 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
2016-05-20T21:10:32.105190Z 0 [ERROR] Failed to initialize plugins.
2016-05-20T21:10:32.105201Z 0 [ERROR] Aborting

The problem is that the installation of the mysql-server package requires a successful launch of the MySQL server. If MySQL cannot start, then the package is not fully installed.