Initialize 'auth' to empty dict on bifrost_configdrives_dynamic

The bifrost-configdrives-dynamic role invocation fails at
the update_facts_from_ironic include, due to initializing the
'auth' fact to None.
Ansible converts None to 'None', but then the openstack argument
spec expects to be a dict, which fails as it cannot convert it.
Setting this to an empty dict fixes the issue.

Change-Id: Ia807db74b6a2f0cc107216270f8538232019c607
Closes-Bug: LP1612586

playbooks/roles/bifrost-configdrives-dynamic/tasks/update_facts_from_ironic.yaml

@@ -18,7 +18,7 @@
 - name: "Collecting node facts"
   os_ironic_facts:
     auth_type: None
-    auth: None
+    auth: {}
     ironic_url: "{{ ironic_url }}"
     uuid: "{{ uuid | default() }}"
     name: "{{ name | default() }}"