Comment 3 for bug 1579708

In my case the problem is a missing /etc/mysql/my.cnf.fallback configuration file.

Possible workaround:

In /var/log/apt/history.log

Start-Date: 2017-01-20 10:42:27
Commandline: aptdaemon role='role-commit-packages' sender=':1.126'
Upgrade: mysql-server-5.7:amd64 (5.7.16-0ubuntu0.16.04.1, 5.7.17-0ubuntu0.16.04.1), mysql-server:amd64 (5.7.16-0ubuntu0.16.04.1, 5.7.17-0ubuntu0.16.04.1), mysql-client:amd64 (5.7.16-0ubuntu0.16.04.1, 5.7.17-0ubuntu0.16.04.1), libmysqlclient20:amd64 (5.7.16-0ubuntu0.16.04.1, 5.7.17-0ubuntu0.16.04.1), libmysqlclient20:i386 (5.7.16-0ubuntu0.16.04.1, 5.7.17-0ubuntu0.16.04.1), mysql-server-core-5.7:amd64 (5.7.16-0ubuntu0.16.04.1, 5.7.17-0ubuntu0.16.04.1)
Error: Sub-process /usr/bin/dpkg returned an error code (1)
End-Date: 2017-01-20 10:42:27

In /var/log/apt/term.log

Log started: 2017-01-20 10:42:27
Setting up mysql-common (5.7.17-0ubuntu0.16.04.1) ...
update-alternatives: error: alternative path /etc/mysql/my.cnf.fallback doesn't exist
dpkg: error processing package mysql-common (--configure):
 subprocess installed post-installation script returned error exit status 2
Errors were encountered while processing:
 mysql-common
Log ended: 2017-01-20 10:42:27

So the problem for me seems to be the missing the /etc/mysql/my.cnf.fallback file.

ll /etc/myql
drwxr-xr-x 4 root root 4096 gen 20 10:59 ./
drwxr-xr-x 189 root root 12288 gen 19 12:44 ../
drwxr-xr-x 2 root root 4096 gen 20 09:43 conf.d/
-rw------- 1 root root 317 nov 19 10:57 debian.cnf
-rwxr-xr-x 1 root root 120 ott 24 17:13 debian-start*
lrwxrwxrwx 1 root root 24 nov 18 21:12 my.cnf -> /etc/alternatives/my.cnf
-rw-r--r-- 1 root root 3028 nov 18 21:27 my.cnf.migrated
-rw-r--r-- 1 root root 682 lug 11 2016 mysql.cnf
drwxr-xr-x 2 root root 4096 nov 18 22:24 mysql.conf.d/

So, because the symlink:

my.cnf -> /etc/alternatives/my.cnf

points back to:

/etc/alternatives/my.cnf -> /etc/mysql/mysql.cnf

i assumed (crossing my fingers) that /etc/mysql/mysql.cnf should be eligible as alternative fallback:

sudo cp /etc/mysql/myslq.cnf /etc/mysql/my.cnf.fallback

Run again apt tool to complete the upgrade.

This worked for me.