need to validate that no_proxy contains a few critical IPs/domains

The following discussion from !288 (merged) should be addressed:

ensure at validation time that no_proxy covers everything that we need it to cover (including 'localhost,172.16.0.0/12,10.0.0.0/8,192.168.0.0/16,.cluster.local.,.cluster.local,.svc' and .sylva -- but taking into account that .sylva should be {{ .Values.cluster.cluster_external_domain }}