Created Node (Un)Assignment Handler

 * Created  handler, validator,
 added url to  handler mapping.
 * Created test for assignment
 and unassignment.
 * Moved role validation from
 fuelclient to assignment validator
 * new handlers used in fuelclient
 * refactored fueclient for new
 assignment and unassignment API

 Closes-bug: #1265036
 Implements: blueprint create-separate-handler-for-node-assignment

Change-Id: Ie1dce1401e0f43cc336662bf29a4e5e72bd80db1
Merge "Created Node (Un)Assignment Handler"