restapi : Missing opertions for snapshots in RESTAPI

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: If8c6d25daf7e716a7c857295b6ceb1741cbaf7f7
Bug-Url: https://bugzilla.redhat.com/867339
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>