Comment 2 for bug 1626931

In the terminal log it only appears to be starting to then being shut down.
But the dmesg holds more hints at the root cause.

There I wonder about:
[ 17.758717] init: Failed to spawn php7.0-fpm main process: unable to execute: No such file or directory
[ 17.988212] init: Failed to spawn mysql main process: unable to execute: No such file or directory

Then further I see several apparmor denies:
[ 7844.489836] type=1400 audit(1474613743.811:67): apparmor="DENIED" operation="open" profile="/usr/sbin/mysqld" name="/etc/mysql/my.cnf.fallback" pid=29019 comm="mysqld" requested_mask="r" denied_mask="r" fsuid=0 ouid=0
[ 7850.111433] type=1400 audit(1474613749.431:68): apparmor="DENIED" operation="open" profile="/usr/sbin/mysqld" name="/etc/mysql/my.cnf.fallback" pid=29061 comm="mysqld" requested_mask="r" denied_mask="r" fsuid=0 ouid=0
[ 7852.177221] type=1400 audit(1474613751.499:69): apparmor="DENIED" operation="open" profile="/usr/sbin/mysqld" name="/etc/mysql/my.cnf.fallback" pid=29086 comm="mysqld" requested_mask="r" denied_mask="r" fsuid=0 ouid=0
[ 7854.201312] type=1400 audit(1474613753.523:70): apparmor="DENIED" operation="open" profile="/usr/sbin/mysqld" name="/etc/mysql/my.cnf.fallback" pid=29111 comm="mysqld" requested_mask="r" denied_mask="r" fsuid=0 ouid=0
[ 7856.265292] type=1400 audit(1474613755.587:71): apparmor="STATUS" operation="profile_replace" profile="unconfined" name="/usr/sbin/mysqld" pid=29171 comm="apparmor_parser"
[ 7856.268621] type=1400 audit(1474613755.591:72): apparmor="DENIED" operation="open" profile="/usr/sbin/mysqld" name="/etc/mysql/my.cnf.fallback" pid=29173 comm="mysqld" requested_mask="r" denied_mask="r" fsuid=0 ouid=0
[ 7856.273266] type=1400 audit(1474613755.595:73): apparmor="DENIED" operation="open" profile="/usr/sbin/mysqld" name="/etc/mysql/my.cnf.fallback" pid=29182 comm="mysqld" requested_mask="r" denied_mask="r" fsuid=0 ouid=0
[ 7856.449151] init: mysql main process (29182) terminated with status 1
[ 7856.449158] init: mysql main process ended, respawning

Only to further down giving up by:
[ 7858.506585] init: mysql respawning too fast, stopped

I wonder if your installation has any custom apparmor or other configuration changes to mysql that could affect this?

I have seen reports that this issue could be avoided by:
apt-get install mysql-community-client
before upgrading the server. I'd wonder but if you could try and verify/falsify that for your case that would be great.

For the sake of checking if that is bug 1421303 showing up again in some way please also share the output of "sudo aa-status"