Porting os-aggregates extensions to API v3 Part 2

This includes a set of fixes and improvements to os-aggregates:
    - eliminate the action method replacing its functionality with wsgi.action
      decorator.
    - Returns explicity and appropriate status codes for each REST action.
    - Improves information error passed to the user.

In the unit test removes one test that in fact was testing the action method
itself instead of _remove_host and which no longer makes sense since action is
removed.

Partially implements blueprint nova-v3-api

Change-Id: Ie737333c6b058f5b57b3e7c75b3d3eda95b80402
Merge "Porting os-aggregates extensions to API v3 Part 2"