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: I21de53a8f9092085beee28124ef2321538e76669
Signed-off-by: Martin Betak <mbetak@redhat.com>