Return mysql-server to default for all ubuntu installs

Change: I97cbede806e5c00363c7174fa1e9f286de96aab6 altered
the logic in lib/databases/mysql from installing mariadb only if
one was using rhel7 to installing it by default unless one was using
rhel6.

Change: Iae9a7a1c09f1fc83573c3926b3470955c244c401
Attempted to fix this, but only does so for precise.

Unless mariadb has become the default, I strongly prefer to keep
standard mysql as the database on ubuntu (precise or otherwise).

Closes-Bug: 1395776

Change-Id: I3afb89ae6d55405313b7219dd6daa24d9ca80f70
Merge "Return mysql-server to default for all ubuntu installs"