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
Um, the validation situation is not good, but I am not sure it is a fault of this PR:
```
$ xmllint --noout --relaxng \
   /local/home-mvidner/github-checkout/yast/yast-installation-control/control/control.rng \
   test/data/dhcp_hostname_true.xml 
test/data/dhcp_hostname_true.xml:15: element dhclient_set_hostname: Relax-NG validity error : Expecting element incomplete_translation_treshold, got dhclient_set_hostname
test/data/dhcp_hostname_true.xml:12: element additional_kernel_parameters: Relax-NG validity error : Element globals has extra content: additional_kernel_parameters
test/data/dhcp_hostname_true.xml:32: element texts: Relax-NG validity error : Expecting an element , got nothing
test/data/dhcp_hostname_true.xml:32: element texts: Relax-NG validity error : Element texts failed to validate content
test/data/dhcp_hostname_true.xml fails to validate
```