This patch enables the ceph mgr prometheus exporter.
If enable_prometheus_ceph_mgr_exporter is set to true,
the ceph mgr prometheus plugin is enabled on the hosts that are part
of the ceph-mgr group, then the exporter is added into the prometheus-server
configuration file.
Change-Id: Ia2f879401e585e6043f69cc5e3ab1a1f72f7f033