ovs: force to secure fail mode by default

The upstream Ansible module seems to pass through a 'None' when
creating the port with safe_mode missing which breaks.  This patch
forces it to the Neutron required mode of 'secure'.

Change-Id: I5a171ee83edcfa152b944e0bc39a90f2a8546916

tasks/providers/ovs_config.yml

@@ -17,6 +17,7 @@
 - name: Setup External Network Provider Bridge
   openvswitch_bridge:
     bridge: "{{ neutron_provider_networks.network_mappings.split(':')[1] }}"
+    fail_mode: secure
     state: present
   when:
     - neutron_services['neutron-l3-agent']['group'] in group_names