gluster: Corrected the canDoAction message for rebalance

Corrected the canDoAction message for gluster volume rebalance command on case
there is a single brick in the volume. The new message is "Cannot
${action} ${type}. Gluster Volume has a single brick."

Change-Id: I449d86daca772a1557e94d9f01cf5a170c749782
Bug-Url: https://bugzilla.redhat.com/1008318
Signed-off-by: Shubhendu Tripathi <shtripat@redhat.com>