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
diff --git a/lib/databases/mysql b/lib/databases/mysql
index 7a444a3..bbf2fd0 100644
--- a/lib/databases/mysql
+++ b/lib/databases/mysql
@@ -142,7 +142,7 @@
     fi
     # Install mysql-server
     if is_ubuntu || is_fedora; then
-        if [[ $DISTRO =~ (rhel6) ]]; then
+        if [[ $DISTRO =~ (rhel6|precise) ]]; then
             install_package mysql-server
         else
             install_package mariadb-server