Fix issue with scale down and overcloud TLS

Adding RootStackName variable to the scale tasks so that
we can reference it instead of the existing environment
variables. This will ensure that the scale down uses the
environment variables from clouds.yaml and get the
OS_CACERT while trying to speak with the overcloud endpoints

Change-Id: Ia8868172fb16b294208ee3d6b03c09442fe39443
Closes: #1913275

deployment/nova/nova-compute-container-puppet.yaml

@@ -717,6 +717,10 @@ parameters:
     tags:
       - role_specific
 
+  RootStackName:
+    description: The name of the stack/plan.
+    type: string
+
 parameter_groups:
 - label: deprecated
   description: |
@@ -1615,14 +1619,7 @@ outputs:
             - container_cli == 'podman'
           tags: down
           environment:
-            OS_USERNAME: admin
-            OS_USER_DOMAIN_NAME: "Default"
-            OS_PROJECT_DOMAIN_NAME: "Default"
-            OS_PROJECT_NAME: admin
-            OS_PASSWORD: { get_param: AdminPassword }
-            OS_AUTH_URL: { get_param: [EndpointMap, KeystoneV3Public, uri] }
-            OS_IDENTITY_API_VERSION: 3
-            OS_AUTH_TYPE: password
+            OS_CLOUD: {get_param: RootStackName}
           block:
             # Some tasks are running from the Undercloud which has
             # the OpenStack clients installed.