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
```DNS.dhcp_hostname``` states if hostname is written into etc/hosts. It has nothing to do with dhclient. Moreover sysconfig (and dhcp leases in this case) is handled by wicked now, so talking about dhclient is confusing.