Add /role_assignments endpoint support to keystoneclient

This blueprint is consequence from a missing feature cited at bug #1246310.

Currently, the keystoneclient doesn’t support calls to the /roles_assignments endpoint (http://api.openstack.org/api-ref-identity.html#Role_Calls), in this way, other OpenStack modules can’t perform operations like fetching all user roles in a project or domain in a single API call (see bug #1278920).

To address this feature, here is a possible implementation plan (keystone client library side):

Create a Assignment Resource class and a AssignmentsManager, only with a list() method. All
other methods implemented by CrudManager would return an error informing the user that it
is not currently supported.

The call would be like: api.keystone.assignments.list()