Adding Metadef Tag support

Adding rest api and db support for CRUD operations on the new
metadef_tags table.

Implements: https://blueprints.launchpad.net/glance/+spec/metadefs-tags
DocImpact

Change-Id: Icfa40555280ce69766381b0abe7ef399b806f6a0
Merge "Adding Metadef Tag support"