snapview-server: register a callback with glusterd to get notifications

* As of now snapview-server is polling (sending rpc requests to glusterd) to
  get the latest list of snapshots at some regular time intervals
  (non configurable). Instead of that register a callback with glusterd so that
  glusterd sends notifications to snapd whenever a snapshot is created/deleted
  and snapview-server can configure itself.

Change-Id: I17a274fd2ab487d030678f0077feb2b0f35e5896
BUG: 1119628
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/8150
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>