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
The production control files do validate:
```
$ xmllint --noout --relaxng yast-installation-control/control/control.rng skelcd-control-*/control/control*.xml 
skelcd-control-openSUSE/control/control.openSUSE-promo.changes.xml validates
skelcd-control-openSUSE/control/control.openSUSE.xml validates
skelcd-control-SLED/control/control.SLED.xml validates
skelcd-control-SLES/control/control.SLES.xml validates
```

So can you please make the test control files valid too? That includes adding the new element to `yast-installation-control/control/control.rng`