Handle registry role running under py3

Also, correct the host_vars filename.  Again.
Also, make sure we run the test on changes to the host_vars filename.

Change-Id: I95fb61531bae677f5c68f4e56ed718da6c507eb9

.zuul.yaml

@@ -495,6 +495,7 @@
     files:
       - .zuul.yaml
       - playbooks/group_vars/registry.yaml
+      - ^playbooks/host_vars/insecure-ci-registry\d+.opendev.org.yaml
       - playbooks/zuul/templates/group_vars/registry.yaml.j2
       - playbooks/roles/registry/
       - testinfra/test_registry.py

playbooks/roles/registry/tasks/main.yaml

@@ -13,7 +13,7 @@
 - name: Install passlib
   package:
     name:
-      - python-passlib
+      - python3-passlib
     state: present
 - name: Write htpassword file
   htpasswd: