Use mysql in Ubuntu 12.04 (precise)

After commit 6d20f090, devstack does not work on Ubuntu 12.04
because Ubuntu 12.04 does not provide mariadb but devstack expects it.

Change-Id: Iae9a7a1c09f1fc83573c3926b3470955c244c401
Merge "Use mysql in Ubuntu 12.04 (precise)"