webadmin: Fix NPE in SnapshotModel

CancelCommand was not properly initialized in SnapshotModel.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1056651
Change-Id: I732132f1f9132e47451fcf02e793d0a2ffc6d11f
Signed-off-by: Martin Betak <mbetak@redhat.com>
Signed-off-by: Daniel Erez <derez@redhat.com>