Migrate to hostnamectl (if possible)

hostnamectl is a tool which allows to configure the hostname.
This patch migrates away from sysconfig towards hostnamectl.

Legacy scripts (like the old UI) are not migrated.

rhbz#882206

Change-Id: I282192ddb27ac4596a3938d2e812cda38384b660
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>