Only add a comma before multiple update repos

When adding repos to update in standalone
and undercloud, only add a comma before the list
of additional repos if a previous update repos
exist.

Change-Id: Ic1568c99e16eecd607f063fe8a95fa8c4aa9ed2b

roles/standalone/tasks/containers.yml

@@ -212,7 +212,8 @@
 
     - name: Create string from list
       set_fact:
-        add_repos_update: ",{{ add_repos_update | join(',') }}"
+        add_repos_update:
+          "{% if standalone_container_prep_update_repo|length >0 %},{% endif %}{{ add_repos_update | join(',') }}"
       when: add_repos_update|length > 0
 
 - name: Load the container params file data

roles/undercloud-deploy/tasks/create-scripts.yml

@@ -129,7 +129,8 @@
 
     - name: Create string from list
       set_fact:
-        add_repos_update: ",{{ add_repos_update | join(',') }}"
+        add_repos_update:
+          "{% if update_containers_repo|length >0 %},{% endif %}{{ add_repos_update | join(',') }}"
       when: add_repos_update|length > 0
 
     - name: Create string from list