Use import_role on persistent-firewall

Previously to persist the filewall we were including the
persistent-firewall role. This seems to occasionally break because the
second invocation of the role (on multinode jobs after setting up the
multinode bridge) fails with an RC of -13 when listing ipv4 iptables
roles. Then when we try to write them to disk the variable is empty.

One thought is that dynamically loading the role multiple times may be
confusing ansible. Use import_role to statically load the role instead
and see if this helps.

Change-Id: I2458f8eb4c2e4638336fa14e436e13b5a2263cce

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

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

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

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