Filter User by project

In version v2.0 keystone allows filtering users by projects
(v2.0/tenants/{tenantId}/users) but in v3 it doesn't.

If you need to list the users belonging to a project in
keystone v3, you must use the role_assignments API,
but it only returns a list of user_ids. To get further
information like login and name, it is necessary to
perform another query, using this id as a parameter,
using the user API.

We create the API call "v3/projects/{project_id}/users",
which will internally make the process above described,
returning information from the users, instead of
only the user_id.

Change-Id: I242c49e3164e876adc2990b8cae50dca82172fe1
Implements: blueprint filter-user-by-project-v3
Merge branch 'filter_user_by_project' of https://github.com/raildo/keystone-1 into filter_user_by_project