Improve attaching/detaching NICS

Created by: garloff

add_cluster-network.sh: Save unnecessary server list calls: We know the name of the -mgmtcluster host. remove_cluster-network.sh: Determine need to delete NIC and determine net and NIC name. Exit if nothing needs to be done. delete_cluster.sh: Call remove_cluster-network.sh, just in case some cleanup is needed. It's harmless if not. If it is, it helps us to ensure that kubectl delete cluster can succeed.

Signed-off-by: Kurt Garloff kurt@garloff.de