Add max_allowed_packet parameter

This patch allows you to set the max_allowed_packet which has a default
of 16MB. This was needed in order to get mysql working for use with the
Puppet Dashboard which requires tuning the max_allowed_packet to at least 32MB.

http://docs.puppetlabs.com/dashboard/manual/1.2/bootstrapping.html#creating-and-configuring-a-mysql-database