Support setting docker registry credentials to the empty string

The use case is that I want to set these credentials via extra vars
like this:

  kayobe overcloud container image build test -e kolla_docker_registry_username=$HARBOR_REGISTRY_USER -e kolla_docker_registry_password=$HARBOR_REGISTRY_PASSWORD

Supporting the empty string means that I don't need to use any
conditional logic.

Change-Id: Ie5c21597266c8e470994eba16f3c2ed2224d0dcb

ansible/container-image-build.yml

@@ -41,8 +41,9 @@
         password: "{{ kolla_docker_registry_password }}"
         reauthorize: yes
       when:
-        - kolla_docker_registry_username is not none
-        - kolla_docker_registry_password is not none
+        # NOTE(wszumski): Switch to truthy filter when min ansible>=2.10.5
+        - kolla_docker_registry_username not in [none, ""]
+        - kolla_docker_registry_password not in [none, ""]
 
     - name: Ensure Kolla container images are built
       shell: