Add newrelic mysql plugin support to stack_commons::newrelic

Just to match "wellformed newrelic license"
https://github.com/newrelic-platform/newrelic_plugins_chef/blob/master/definitions/verify_license_key.rb#L2

I've commented on my commit https://github.com/rackspace-cookbooks/stack_commons/commit/e11a46ac6dccc3456b9638269a4e213acd8f7b5e