oslo project provide jsonutils, and the others project use it now,
this PS to update json moudule to oslo jsonutils for consistency.
Change-Id: I51bcac143d3750aa991aeed6e95e011389f011d8
* It merges the freezer-api tempest plugin into freezer tempest
plugin by moving freezer and freezer-api tempest tests in
freezer_tempest_plugin/tests/freezer and
freezer_tempest_plugin/tests/freezer_api.
* clients.py is moved under main root folder.
Change-Id: I66cc2507b0bbd9dda9d6279f9b8d74c546d1b0a6