Revert "Use import_role on persistent-firewall"

This reverts commit 46b7b6e1c9.

This didn't end up changing the incidence of the iptables-save command
task failures.

Change-Id: I02e725d7330bc9b438a9864ea49510cca7fee524

roles/multi-node-bridge/tasks/main.yaml

@@ -18,5 +18,5 @@
   static: no
 
 - name: Persist iptables rules
-  import_role:
+  include_role:
     name: persistent-firewall

roles/multi-node-firewall/tasks/main.yaml

@@ -48,5 +48,5 @@
   with_items: "{{ ipv6_addresses }}"
 
 - name: Persist iptables rules
-  import_role:
+  include_role:
     name: persistent-firewall