linters: rename loop variable

In this repo we name the loop variables.  Although this is a test
playbook, it's good for consistency.  This is picked up by a later
version of ansible-lint.  This should have no operational change.

Change-Id: I084a1e8515fe1fda039190fe6518512ebf03217e

test-playbooks/registry/roles/run-test-intermediate-registry/tasks/main.yaml

@@ -2,10 +2,12 @@
   become: true
   file:
     state: directory
-    path: "/var/registry/{{ item }}"
+    path: "/var/registry/{{ zj_reg_dir }}"
   loop:
     - certs
     - auth
+  loop_control:
+    loop_var: zj_reg_dir
 - name: Install python packages
   become: true
   package: