Glance scrubber should page thru images from registry

If the registry is limited in it's page size when returning the
list of images, the scrubber will only receive the first page.
This patch enables the scrubber to retrieve all the pages of
images.

The new tests are implemented with mock rather than mox. There
is another patch (https://review.openstack.org/#/c/111995) to
convert the existing code in test_scrubber to use mock.

Change-Id: I106f679be96bd39167337c4e27f4046f90d7d7ff
Closes-Bug: 1280343
Merge "Glance scrubber should page thru images from registry"
Remove eventlet_hub option

The option has a default value that is not portable, and therefore the
test fails on OS like Darwin. There's actually no need to specify any
hub – no other OpenStack projects as that option – as in theory Eventlet
is smart enough to pick the best implementation available anyway.

However, since Glance only works with poll or select as a hub, let's try
them in that order. That avoids a user selecting things like 'epolls'
and expecting Glance to work, whereas it would fail.

Also remove a useless cleanup() call in the failing test as the
start_servers() method already does a cleanup itself.

Change-Id: Ia7765151ebb00cdf01e96cf39f3242899d358772