infra/image system-services: Fix DNS forwarder

The DNS forwarder is set while deploying the IPA server. This forwarder
might not be correct later on.

The old /etc/resolv.conf is copied to /etc/resolv.conf.fixnet by the
fixnet service and later on the fixipaip service is trying to get the
nameserver from the copied file. If the retrieval failed of if the
namesever is 127.0.0.1, then the nameserver will be set to 8.8.8.8.

After fixing the IP addresses for the forward and reverse zone, also the
forwarder is set for the dnsserver "${HOSTNAME}".