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
At a first sight, it makes a lot of sense. The point here is that, with the current design, we need to call it from ```lib/network/network_autoconfiguration.rb```. Everything could be refactored to ensure that only ```DNS``` is responsible for the value of ```dhcp_hostname``` during installation, but it sounds risky to me right now. Of course, I would gladly add FIXMEs.