Image not downloaded without restricted property

If restrict download rule is configured in policy.json, and image is
added without protected property mentioned in "restricted" rule, then
normal users (other than admin) are not able to download the image.

Added logic in policy.py, to allow normal user to download image
without restricted property.

https://review.openstack.org/#/c/127923/
Above patch will fix the issue for master, but as it is too large to
be back-ported to stable/juno, Nikhil recommended to push only minimal
changes that fixes this issue after discussing it with the release manager.

In this patch, I have copied required changes from oslo-incubator
policy module from commit 33533b0d97639ed828a2fd5e874f16eb1ecfeaa4.
Note: In juno release, oslo-incubator policy module was not synced
with glance completely. 

Closes-Bug: #1387973
Change-Id: Ib85901b073c85ede7da2e9c7333426c8b8de3bb6
Merge "Image not downloaded without restricted property" into stable/juno