Use control.xml to set DNS.dhcp_hostname (bnc#870896)

See also related pull requests in https://github.com/ancorgs/skelcd-control-SLED and https://github.com/ancorgs/skelcd-control-SLES
REQ: this is completely wrong. DNS#dhcp_hostname is not hostname. It is used as a flag whenever the hostname should be written also into /etc/hosts