Comment 9 for bug 1428571

I've managed to resolve this issue by removing mysql-apt-config.

My /var/log/dpkg.log said:
2015-03-18 16:02:41 startup packages configure
2015-03-18 16:02:41 configure mysql-apt-config:all 0.3.3-1ubuntu14.04 <none>
2015-03-18 16:02:41 status half-configured mysql-apt-config:all 0.3.3-1ubuntu14.04

mysql-apt-config was half-configured (because I had to kill it) so I couldn't use apt-get because it would just tell me to:
'sudo dpkg --configure -a' (which would not work)

I first tried to remove mysql-apt-config using (don't do this, a better solution is offered in the linked thread bellow):
sudo dpkg --remove --force-remove-reinstreq mysql-apt-config

as I couldn't use apt-get or aptitude, but this just managed to bork up my system somewhat as I couldn't properly update or re-install mysql, but at least I was able to use apt-get/aptitude now.

On trying to reinstall mysql-server-5.6:
dpkg: error processing archive /var/cache/apt/archives/mysql-client-5.6_5.6.19-0ubuntu0.14.04.1_amd64.deb (--unpack):
 trying to overwrite '/usr/share/man/man1/mysql_config_editor.1.gz', which is also in package libmysqlclient-dev 5.6.23-1ubuntu14.04
dpkg-deb: error: subprocess paste was killed by signal (Broken pipe)
Processing triggers for man-db (2.6.7.1-1) ...
Errors were encountered while processing:
 /var/cache/apt/archives/mysql-client-5.6_5.6.19-0ubuntu0.14.04.1_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
A package failed to install. Trying to recover:
Setting up mysql-client-core-5.6 (5.6.19-0ubuntu0.14.04.1) ...
dpkg: dependency problems prevent configuration of mysql-server-5.6:
 mysql-server-5.6 depends on mysql-client-5.6 (>= 5.6.19-0ubuntu0.14.04.1); however:
  Package mysql-client-5.6 is not installed.

dpkg: error processing package mysql-server-5.6 (--configure):
 dependency problems - leaving unconfigured
Setting up mysql-common-5.6 (5.6.19-0ubuntu0.14.04.1) ...
Setting up mysql-server-core-5.6 (5.6.19-0ubuntu0.14.04.1) ...
Errors were encountered while processing:
 mysql-server-5.6

THE SOLUTION:
Finally the accepted answer here helped me resolve the issue for good:
https://stackoverflow.com/questions/27154067/could-not-select-ok-in-mysql-apt-config-ubuntu-14-04

If you need to reinstall mysql-server you may want to purge mysql-common, it doesn't seem to get removed automatically along with mysql-server/client, this was also giving me some issues.

I've removed mysql-apt-config from my system (it doesn't seem to be really needed, and I hope this will help avoid a similar situation in the future), reinstalled msyql-server-5.6 and everything is working fine now, but I don't really get the point of all this, why would I ever want to choose between mysql 5.6 and 5.7-dmr if I explicitly installed 5.6. There should be a separate package for 5.7 for imho.