restapi: Introduced statistics for glustervolumes

Introduced statistics under glustervolumes resource. This would be used
for listing the volume capacity details like used, free and total
capacity of the volume.

Change-Id: I6b17170f403c6ac320829b23b8e2d03b537b8ff2
Signed-off-by: Shubhendu Tripathi <shtripat@redhat.com>