Revert the instance IP conflicts

Since instances can live in different VLANs from nodes (especially in
routed mode), based on the 'link' parameter, we shouldn't always
restrict having duplicate IPs. Thus we only check the node IPs/cluster
IP for now.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>