network: Only update DNS config, not everything

Previously the complete network configuration (and it's configuration
files) were rewritten from scratch when ovirtfunctions.configure_dns()
got triggered, which happens at every boot.
This also lead to custom ifcfg-* scripts getting removed.
This is now fixed and only the DNS related transaction is run.

Change-Id: Ic1fcc6946010903d851d51e9cdb4923cd6c7ab39
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>