This ERROR should be handled prior to trying to iteration attempt.

2018-09-28 11:38:10.124 3320 ERROR cinder.scheduler.filter_scheduler [req-e8cb2375-a6d3-4afb-a146-21be0d1c8777 d6c1ad4f37c74ffd862f5c22cc869ce5 0896f504b30e4522b605d4f3c784dca1 - default default] Error scheduling None from last vol-service: hostgroup@scaleio#OPS:Standard : [u'Traceback (most recent call last):\n', u' File "/usr/lib/python2.7/site-packages/taskflow/engines/action_engine/executor.py", line 53, in _execute_task\n result = task.execute(**arguments)\n', u' File "/usr/lib/python2.7/site-packages/cinder/volume/flows/manager/create_volume.py", line 951, in execute\n **volume_spec)\n', u' File "/usr/lib/python2.7/site-packages/cinder/volume/flows/manager/create_volume.py", line 898, in _create_from_image\n image_service)\n', u' File "<string>", line 2, in _create_from_image_cache_or_download\n', u' File "/usr/lib/python2.7/site-packages/cinder/coordination.py", line 176, in _synchronized\n return f(*a, **k)\n', u' File "/usr/lib/python2.7/site-packages/cinder/volume/flows/manager/create_volume.py", line 779, in _create_from_image_cache_or_download\n backend_name) as tmp_image:\n', u' File "/usr/lib64/python2.7/contextlib.py", line 17, in __enter__\n return self.gen.next()\n', u' File "/usr/lib/python2.7/site-packages/cinder/image/image_utils.py", line 670, in fetch\n fetch_verify_image(context, image_service, image_id, tmp)\n', u' File "/usr/lib/python2.7/site-packages/cinder/image/image_utils.py", line 288, in fetch_verify_image\n None, None)\n', u' File "/usr/lib/python2.7/site-packages/cinder/image/image_utils.py", line 223, in fetch\n image_service.download(context, image_id, image_file)\n', u' File "/usr/lib/python2.7/site-packages/cinder/image/glance.py", line 341, in download\n for chunk in image_chunks:\n', u"TypeError: 'NoneType' object is not iterable\n"]

Also opened upstream: https://bugs.launchpad.net/cinder/+bug/1798147
Proposed fix : https://review.openstack.org/#/c/611081/