engine: Added new canDoAction check for gluster

In case the server being added is already part of an existing gluster
cluster, canDoAction should fail with an appropriate error message.

Also added new test cases for gluster related canDoAction errors.

Change-Id: I81427fa940bf1edabf316acebc428becac0171f8
Bug-Url: https://bugzilla.redhat.com/916982
Signed-off-by: Shireesh Anjal <sanjal@redhat.com>