remove-registry-tag: no_log assert

Just to be sure, add no_log here.  Since the actual authenticated call
is no_log too; if this fails you at least know you didn't get the
credentials layout right which narrows down the problems the actual
call might be facing.

Change-Id: Ic7174c10f4e693f6a2c0554dc66ea22fd28d678f

roles/remove-registry-tag/tasks/docker.yaml

@@ -1,4 +1,5 @@
 - name: Ensure registry token is set
+  no_log: true
   assert:
     that: >
         (container_registry_credentials[_registry].username is defined) and