gluster: Improved gluster validation in add host

When checking if the host is part of another gluster cluster,
- Ignore if the command (gluster peer status) fails, to handle cases
  where glusterfs is not installed or glusterd is down
- Do not perform the check if the cluster in which the host is being
  added is currently empty i.e. when the host being added will be the
  first host of the cluster.

Change-Id: I06a2ee4621b175aa70f1ccdcf7a740d830494acf
Bug-Url: https://bugzilla.redhat.com/956077
Signed-off-by: Shireesh Anjal <sanjal@redhat.com>