Better handling of mysql datadir/volume

Assume a volume mounted on `/var/lib/mysql`. 
 1- The volume is empty : a new database will be created as the one added by apt-get will have been replaced by an empty dir.
 2- The volume is not empty (-> already populated with a db) : we just run mysql_safe 

Problems: 
If nothing is mounted on `/var/lib/mysql` and the data is stored in the container (oh god why ?), no `admin` account will be created (as the dir will not be empty). No `admin` account : no remote access, no fun.