Add centralauth to puppet db_config.yaml

Add the CENTRALAUTH_ENGINE_URL field to /etc/wikimetrics/db_config.yaml
template. For this centralauth parameters are added to the class,
and used in the template.

Also, add centralauth_testing database to be created.

Also add centralauth_testing database to be created under wikimetrics role.

Change-Id: Id7d38482636a05295d8d44b90fe91d19a3f0897a