Validate Grafana is present for dashboarder service

Check if MON can resolve a grafana service.
This is currently useful in the transition to K8s,
or in OSM distros with absent Grafana.

Fixes Bug 932

Change-Id: I3b5ebf3d8ee3397f16793cd730e7e1fa5f6676d4
Signed-off-by: lavado <glavado@whitestack.com>