tests: Fix throttling of Docker pulls

fixes #809

.ci/ansible_install.py

@@ -16,9 +16,9 @@ batches = [
     ]
 ]
 
-batches.extend(
-    ['docker pull %s' % (ci_lib.image_for_distro(distro),), 'sleep 1']
+batches.append(ci_lib.throttle(
+    'docker pull %s' % (ci_lib.image_for_distro(distro),)
     for distro in ci_lib.DISTROS
-)
+))
 
 ci_lib.run_batches(batches)

.ci/ci_lib.py

@@ -129,6 +129,20 @@ def combine(batch):
     ))
 
 
+def throttle(batch, pause=1):
+    """
+    Add pauses between commands in a batch
+
+    >>> throttle(['echo foo', 'echo bar', 'echo baz'])
+    ['echo foo', 'sleep 1', 'echo bar', 'sleep 1', 'echo baz']
+    """
+    def _with_pause(batch, pause):
+        for cmd in batch:
+            yield cmd
+            yield 'sleep %i' % (pause,)
+    return list(_with_pause(batch, pause))[:-1]
+
+
 def run_batches(batches):
     """ Run shell commands grouped into batches, showing an execution trace.