Add extensions for API v2 controllers

Controllers (and API endpoints) should be able to be selected using
configuration flags, as we do for plugins

TODO:
 - Extra Hooks to be configured using stevedore

Implements blueprint: pecan-wsme
Implements blueprint: api-extensions

Change-Id: I12f1bc1accbc870aacb0d1e08c9e0d08223c4b18
Merge "Add extensions for API v2 controllers"