Refactor mysql for upstream modules

Temporary workaround. Will not be needed after "mysql" module sync
with upstream code.

New OpenStack puppet modules (v4.0.0) require "mysql::config"
class. We need to provide it.

Implements: blueprint merge-openstack-puppet-modules

Change-Id: I5f928113a45991f7f0eb5b6a2e5e6a83f3c08a7b
Merge "Refactor mysql for upstream modules"