Comment 10 for bug 1573279

Hi !
I had the same issue and fixed it almost like Rick except I had to remove all the possible my.cnf "alternatives".
Before that I purged mysql-server-5.7.
Then I was able to reinstall it. I checked again all my databases and everything was OK.

sudo apt-get purge mysql-server-5.7
cd /etc/mysql
sudo rm my.cnf my.cnf.fallback my.cnf.migrated my.cnf.wba.bak /etc/alternatives/my.cnf
sudo apt-get install mysql-server
sudo mysqlcheck --all-databases -p

I think I could just have reconfigured all (sudo dpkg --configure -a) instead of purging sql-server-5.7 but after multiple trial and errors I thought it was safer.