Small corrections in image_uploader
Small corrections in image_uploader

This patch addresses the small nits and comments listed in
https://review.openstack.org/639037

It also addresses some scoping issues detected afterward.

Finally, it corrects some logic issues - the no_verify_registries
should not influence on the URL building, since it affects an
option inside the requests.Session option.

Change-Id: I1eba8b982dab777e44da2b75666ea6aa8fa38c5e

openstack/tripleo-common,openstack/tripleo-common