Implement registry API v2

This patch implements registry's API v2 based on the blueprint
registry-api-v2.

Major changes:
    * The new registry API is implemented using a small rpc protocol
      over http. Resources must be registered and the exposed methods
      will be automatically discovered (they must start with rpc_).
    * Registry's API now returns data as returned by the db-api. The
      idea behind this is to provide a db-like api that can be used as a
      db-driver leaving to glance-api the responsibility for cleaning
      up what will go out to the client.

Minor changes:
    * Split unit/v1/test_api.py in 2 separate test files (for
        consistency with v2 unit)
    * Added test_registry to unit/v2/

Implements blueprint registry-api-v2

Change-Id: Ia9f6eb60f52bb093e807a8a89d4df89fb80c4336