Download each cached image once.

Nodepool's devstack image cacher was redownloading every image one time
per devstack branch. We only need to download the image once if the
image name doesn't change.

Change-Id: Ib1910c34b8479ebad740a915fcc9297205cbe387

modules/openstack_project/files/nodepool/scripts/cache_devstack.py

@@ -106,7 +106,7 @@ def main():
     distribution = sys.argv[1]
 
     branches = local_prep(distribution)
-    image_filenames = {}
+    image_filenames = []
     for branch_data in branches:
         if branch_data.get('debs'):
             run_local(['sudo', 'apt-get', '-y', '-d', 'install'] +
@@ -121,6 +121,7 @@ def main():
             fname = url.split('/')[-1]
             if fname in image_filenames:
                 continue
+            image_filenames.append(fname)
             run_local(['wget', '-nv', '-c', url,
                        '-O', os.path.join(CACHEDIR, fname)])