Add new metric-service interface to support use of
Gnocchi as a storage backend for resource and metric
data.

Configure ceilometer to use the gnocchi dispatcher
in the event that ceilometer is related to gnocchi.
This has the side effect of disabling the ceilometer
API - Aodh and Gnocchi API's should be used directly
in this deployment topology.

Note that Gnocchi is only supported in OpenStack
Mitaka or later; 'metrics-service' is added to the
required interfaces configuration as an alternative
to 'mongodb' for >= Mitaka.

Change-Id: Ia31dfefd5efa3fb5ec2ba5d132ee865c567bd8df