This brings the Images API Reference to the glance tree as part of
the effort described here:
https://wiki.openstack.org/wiki/Documentation/Migrate#API_Reference_Plan
This patch includes the Metadefs API reference, with the content
revised and corrected.
Change-Id: I8c10893e92edb7c608b58d33539ab57ab4f57d3e
Closes-bug: #1599314