Ensure python-mysqldb is installed before MySQL db_sync

Ensure python-mysqldb is installed  before MySQL db_sync

When running MySQL for storage backend, we need to make sure
python-mysqldb is installed *before* the db_sync command, otherwise it
will fail:
"ImportError: No module named MySQLdb".

This patch ensure the ordering to ensure puppetlabs-mysql installed the
package before puppet-ceilometer tries to synchronize the MySQLdatabase.

Change-Id: Iae7c8f24fda32357a61ffa3b8597dd3413c303cb