Support for glance metadata definitions

There is a blueprint which is being implemented in glance that will provide whole new functionality - metadata definitions catalog.
Blueprint: https://review.openstack.org/#/c/98554 (accepted)
Code: https://review.openstack.org/#/c/111483 (accepted)
The blueprint introduces new command glance-manage db load_metadefs that loads metadata definitions from json files and inserts them into the glance database.

To initialize metadata definitions catalog devstack installation process should copy every json file from 'glance_repo/etc/metadefs' directory to '/etc/glance/metadefs/' and call 'glance-manage db load_metadefs'.