Add role assignments list support to identity v3

The assignments manager and its test class were created.
Some fake stubs were also added on the fakes.py module.

The "openstack role assignment list" command was created.

Change-Id: Iae94f4fee608ea3e09ff38961ad22edc38efb89c
Implements: blueprint roles-assignment-list
Closes-Bug: 1246310
Merge "Add role assignments list support to identity v3"