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
Not so sure, the documentation of ```ProductFeatures.GetFeature``` says ```@return [Object]``` and calling ```empty?``` will only work on Ruby's enumerables and similar classes (strings, arrays, hashes, etc.). For example, it will raise an exception if ```GetFeature``` returns ```nil```. I'll try to improve the comparison, but ```empty?``` sounds risky unless a string or an array is ensured by the called method.