Add check for nova-api-metadata

This adds a new maas_local check for the nova-api-metdata service. We
also change the alarm status message for the nova_api_local_status
alarm.

This finishes off issue https://github.com/rcbops/ansible-lxc-rpc/issues/67 but cannot be merged until https://github.com/rcbops/rpc-maas/pull/100 is merged.