Refactoring get_grant() and list_grants()

Refactoring those methods to receivea list of parents as parameters.
Merge branch 'list_inherited_roles_backup' of https://github.com/rodrigods/keystone into list_inherited_roles_backup

Conflicts:
	keystone/assignment/backends/kvs.py
	keystone/assignment/backends/sql.py
	keystone/assignment/controllers.py
	keystone/assignment/core.py
	keystone/tests/test_backend.py