Fix resolv.conf search/nameserver

* Ensure additional nameserver/search, if defined as vars.
* Don't backup changed dhclient hooks as they are going to be
  executed by dhclient as well, which is not what we want.
* For debian OS family only:
- Rename nodnsupdate hook the resolvconf hook to be sourced always
before it.
- Ensure dhclient restarted via network restart to apply the
nodnsupdate hook.
* For rhel OS family, the fix TBD, it doesn't work the same way.

Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>