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
Fixme added at https://github.com/yast/yast-network/pull/259/files#diff-fd690548c8d9ccc25675e4b0e82a0b49R86