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: the test doesn't test Installation, it only tests a running system (ProductFeatures/Import). During installation, settings come directly from control file (control.xml) which you have changed in other PR anyway.