Tag Catalog Support For Metadata Definitions

This blueprint adds the basic tag catalog back after it was deferred from Juno
due to time constraints. The original, approved spec included supporting tag
libraries in the metadata definitions catalog. However, at the end of
Juno, the original spec was updated to remove tags since they weren't
implemented.

This spec is actually a reduction of the tag concepts in the original
approved Juno spec. In the original Juno spec, tags had a dynamic hierarchy
capability.  This spec does not include the tag hierarchy in order to
simplify this spec. That aspect of tags will be deferred to a later spec.

The implemented juno spec is here for reference:

https://github.com/openstack/glance-specs/blob/master/specs/juno/metadata-schema-catalog.rst

Change-Id: I2eab7dc2c23894897bb053926baeac917dbd2bfe
Co-Authored-By: Travis Tripp <travis.tripp@hp.com>
Co-Authored-By: Wayne Okuma <wayne.okuma@hp.com>
Co-Authored-By: Lakshmi N Sampath <lakshmi.sampath@hp.com>
Co-Authored-By: murali sundar <murali.sundar@intel.com>
Merge "Tag Catalog Support For Metadata Definitions"