decouple mysql setup from gerrit module

This commit moves the MySQL configuration from the gerrit puppet
module into a seperate mysql puppet module.  The purpose of
this change is to allow us to more easily customise gerrit's
mysql configuration for each instance of gerrit that we deploy..

Partial-Bug: 1083101
Change-Id: Ibcc31b3fce8af54229fd4de69a49842ac1c428ae
Merge "decouple mysql setup from gerrit module"