tests: Throttle docker pulls for Ansible tests

I hope this will prevent AWS Elastic Container Registry returning 429:
Too Many Requests error, due to the burst of activity from multiple VMs,
when a CI build begins.

.ci/ansible_install.py

@@ -17,7 +17,7 @@ batches = [
 ]
 
 batches.extend(
-    ['docker pull %s' % (ci_lib.image_for_distro(distro),)]
+    ['docker pull %s' % (ci_lib.image_for_distro(distro),), 'sleep 1']
     for distro in ci_lib.DISTROS
 )