Install openswitch and firewall if need a bridge only

In case of nodeset which doesn't have peers and switch there is
no need to install openswitch and firewall rules, because nothing
will be installed. Let's skip it if no need.
Change-Id: I98cf5ec390ee22e538baa076c9ab87eea6a44c9e

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

@@ -4,6 +4,9 @@
 - include: common.yaml
   environment:
     PATH: "{{ ansible_env.PATH }}:/sbin:/usr/sbin"
+  when: >-
+    inventory_hostname in groups['switch'] or
+    inventory_hostname in groups['peers']
 
 - include: switch.yaml
   environment:
@@ -20,3 +23,6 @@
 - name: Persist iptables rules
   include_role:
     name: persistent-firewall
+  when: >-
+    inventory_hostname in groups['switch'] or
+    inventory_hostname in groups['peers']