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
This just mimics the name from ```/etc/sysconfig/network/dhcp```, where ```DHCLIENT_X``` is still used all along the file. Should we diverge from sysconfig names in search of correctness?