Pluggable Database API for Artifact Storage

The Glance Artifact API Service stores the information about its
artifacts in the main Glance database but in its own set of independent tables,
unrelated to the ones being used by Glance Images API Service. This set of
tables is designed to be artifact-type independent, i.e. to be able to store
the information about all the artifacts regardless of their type and the
version of that types.

However, in some cases such design may be inefficient. This spec addresses this
by providing more capabilities for artifact type developers or deployers.