engine: Remove update functionality from AttachNetworkToCluster

The command is no longer used to update networks that are already
attached to the cluster, so the update functionality can be removed
and the verification that the network didn't exist in the cluster can
be put back.

Change-Id: I7aae709dfa0f55ab78d9c1f3caddf75c9d27deb9
Signed-off-by: Lior Vernia <lvernia@redhat.com>