Add OSSN-0015 - Glance allows non-admin users to create public images

This adds OSSN-0015, which covers an issue related to Glance's default
policy allowing all users to publicize images. This can allow a user
to upload a malicious image in an attempt to attack other users.

Related-Bug: 1313746

Change-Id: Ida7519192a5b77730e4fffa7956978252a3d4c1e