Rework DNS stack to meet hostnet pods needs

* For Debian/RedHat OS families (with NetworkManager/dhclient/resolvconf
  optionally enabled) prepend /etc/resolv.conf with required nameservers,
  options, and supersede domain and search domains via the dhclient/resolvconf
  hooks.

* Drop (z)nodnsupdate dhclient hook and re-implement it to complement the
  resolvconf -u command, which is distro/cloud provider specific.
  Update docs as well.

* Enable network restart to apply and persist changes and simplify handlers
  to rely on network restart only. This fixes DNS resolve for hostnet K8s
  pods for Red Hat OS family. Skip network restart for canal/calico plugins,
  unless https://github.com/projectcalico/felix/issues/1185 fixed.

* Replace linefiles line plus with_items to block mode as it's faster.

Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>
Co-authored-by: Matthew Mosesohn <mmosesohn@mirantis.com>

docs/dns-stack.md

@@ -38,6 +38,17 @@ or `8.8.8.8`. And domain is set to the default ``dns_domain`` value as well.
 Later, the nameservers will be reconfigured to the DNS service IP that Kargo
 configures for K8s cluster.
 
+Also note, existing records will be purged from the `/etc/resolv.conf`,
+including base/head/cloud-init config files and those that come from dhclient.
+This is required for hostnet pods networking and for [kubelet to not exceed search domains
+limits](https://github.com/kubernetes/kubernetes/issues/9229).
+
+New search, nameserver records and options will be defined from the aforementioned vars:
+* Via resolvconf's head file, if resolvconf installed.
+* Via dhclient's DNS update hook.
+* Via cloud-init (CoreOS only).
+* Statically in the `/etc/resolv.conf`, if none of above is applicable.
+
 DNS configuration details
 -------------------------