glusterd/snapshot: Snapshot should be deactivated when it is created

By default snapshot should be deactivated and this should be a
configurable option.

This behaviour can be configured by the command below:
gluster snapshot config activate-on-create <enable|disable>

Change-Id: I1911595c32beed43bb2fca4bf99f0d264b422513
BUG: 1157991
Signed-off-by: vmallika <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/8985
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>