adds support for gluster backend configuration

two new config options are added: CONFIG_CINDER_BACKEND and
CONFIG_CINDER_GLUSTER_MOUNTS

Change-Id: I9a24d52104fea06fa7fcbbd14ccdaf8a3c17a45a
Removing needless slash from 2 usage strings

Also correcting a typo ('s/activtion/activation/').

Change-Id: I878fdde5612c4e91eb609290fefd51946d215ac8
Merge "adds support for gluster backend configuration"
Merge "Removing needless slash from 2 usage strings"