Fix service_name package_gpg_key key_source and docs

* Reuse package_gpg_key for apt as well
* Update/add docs for package_gpg_key, key_source
* Fix key_source usage in repo::apt
* Make the title for rabbitmq-service the same as its name and
  update the specs as well.
  We need this change, due to custom providers being introduced
  might require some additional manipulations with service title
  as well. For example, systemd, system V, upstart OS-aware
  services must be disabled for pacemaker provider and that
  would require redefining ensure/enable for service by its title.

Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>