Project Groups API

The Project Groups API is available at the following endpoints:
* GET /v1/project_groups[/<pg_id>]
* POST /v1/project_groups
* PUT /v1/project_groups/<pg_id>
* GET /v1/project_groups/<pg_id>/projects
* PUT /v1/project_groups/<pg_id>/projects/<project_id>
* DELETE /v1/project_groups/<pg_id>/projects/<project_id>

The GET methods are available to all users. All other methods require
superuser privileges.

Change-Id: Icdd626b0dec0ab34f7fd1f72c7c3edae182daae1
Merge "Project Groups API"