gluster: rename gluster volumes to volumes in remove brick start event log

Renaming 'Gluster Volumes' to 'Volume' in remove brick start event log
messages.

Change-Id: If172ab0d56344192ade014cefb54a3595a91fbeb
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1022516
Signed-off-by: Ramesh Nachimuthu <rnachimu@redhat.com>