restapi : Missing opertions for snapshots in RESTAPI(#867339)

In RESTAPI some opertions for snapshot are missing which are
available in webadmin.

This patch adds preview, commit and undo operations to the
snapshot resource.

Change-Id: If2bbf6ba2c3f71230d3fdd98ca60c0c3d0f902fb
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=867339
Signed-off-by: Ravi Nori <rnori@redhat.com>