gluster: Overriding getVdsGroup in GlusterHookCommandBase

Overriding getVdsGroup in GlusterHookCommandBase to
make sure that vds group is set from hook's cluster id
and is not null for action messages and canDoAction

Change-Id: I669b0039bc3bc5f8edfe5f1292683c5c7e48b4e9
Signed-off-by: Sahina Bose <sabose@redhat.com>