glusterd/snapshot: Activation and De-activation of snapshot

Previously, snapshots by default were activated on creation and there was
no option to activate or deactivate them on demand.

This will allow the user to activate and deactivate on demand.
The CLI goes as follows

1) Activate the snap using a command "gluster snapshot activate <snapname> [force]"
2) Deactivate the snap using a command "gluster snapshot deactivate <snapname>"

Note: Even now the snapshot will be activated during creation.

Change-Id: I0946d800780f26c63fa1fcaf29aabc900140448f
BUG: 1061685
Signed-off-by: Joseph Fernandes <josferna@redhat.com>
Reviewed-on: http://review.gluster.org/7476
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijaikumar Mallikarjuna <vmallika@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>