nftables: Do NOT purge iptables on docker hosts

  docker-ce depends on iptables, and therefore purging iptables will drag
  docker away with it. D'oh!

Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>

nftables/init.sls

@@ -24,4 +24,6 @@ purge-iptables:
   pkg.purged:
     - pkgs:
       - iptables-persistent
+{%- if not 'docker' in salt['pillar.get']('nodes:' ~ grains['id'] ~ ':roles', []) %}
       - iptables
+{%- endif %}