Add /role_assignments endpoint support

This patch adds role assignments list support
to keystoneclient.

Created RoleAssignment resource and RoleAssignmentManager
classes. RoleAssignmentManager only implements the list()
method, the other inherited methods from base.CrudManager
raises a MethodNotImplemented error with customized messages.

This bp is complimented with the OSC part:
https://blueprints.launchpad.net/python-openstackclient/+spec/roles-assignment-list

Change-Id: I164b58b67ff42320238e943ddfa9d0a8aadd0a6d
Implements: blueprint roles-assignment-support
Closes-Bug: #1246310
Merge "Add /role_assignments endpoint support"