Use loop_control for haproxy keystone back-end enablement

In I17dafb283e41ce05083ae4adb3a325aaca0253dd we switched the loop
control specification from in the common tasks to the parent task,
but forgot to implement the loop control for the task which enables
the back-end again later. This patch corrects that.

Change-Id: I0d7e4804bdc92ffe7a679060a686e684c01fcd1b
(cherry picked from commit 8d8755be86)

playbooks/os-keystone-install.yml

@@ -134,9 +134,11 @@
     # to available again.
     - include: common-tasks/haproxy-endpoint-manage.yml
       vars:
-        haproxy_backend: "{{ item }}"
+        haproxy_backend: "{{ backend_name }}"
         haproxy_state: enabled
       when: "{{ groups['keystone_all'] | length > 1 }}"
+      loop_control:
+        loop_var: backend_name
       with_items:
         - "keystone_service-back"
         - "keystone_admin-back"