Separate glance metadef plugin to use native api

The metadata definition service in Glance provides a common API for vendors, admins, services, and users to meaningfully define available key / value pair metadata that can be used on different types of resources (images, artifacts, volumes, flavors, aggregates, etc). A definition includes a property’s key, its description, its constraints, and the resource types to which it can be associated

See:
  - http://docs.openstack.org/developer/glance/metadefs-concepts.html
  - http://docs.openstack.org/developer/glance/glancemetadefcatalogapi.html

The catalog is typically used by UI's like Horizon to present the user with a list of possible tags and properties to use on their instances. The Glance API provides helper functions to help provides some of the property resolution (properties are reused across resource types, but have different prefixes). In searchlight, we will want to investigate adding this additional information. Initially, we just want to be able to gain access to the raw metadefs via elastic search queries. We then will want to be able to provide queries that perform additional ease of use.