Include Metadata Defs Concepts in Dev Docs

The http://docs.openstack.org/developer/glance/
site currently doesn't include the Juno Metadata
Definitions concepts.  This patch adds
an overview of the concepts to this site.

This provides a synopis of the concepts in:
https://github.com/openstack/glance-specs/blob/master/specs/juno/metadata-schema-catalog.rst

DocImpact
Closes-Bug: 1367432
Related-Bug: 1367908
Related-Bug: 1363615
Related-Bug: 1366286
Related-Bug: 1363383
Change-Id: Iad4d388cbbf2f63fa243d04d35032de0cb0bc1b4
(cherry picked from commit 0aab5e271667990afb1a2522c7c9c61fa7211e0b)
Merge "Include Metadata Defs Concepts in Dev Docs" into proposed/juno