Replacement mox by mock

Replace mox testing library by mock one in:
- glance/tests/unit/common/test_rpc.py,
- glance/tests/unit/test_filesystem_store.py,
- glance/tests/unit/v1/test_registry_client.py,
- glance/tests/unit/v1/test_upload_utils.py,
- glance/tests/unit/v2/test_registry_client.py

Partially Implements: blueprint replace-mox-with-mock

Change-Id: Idbdec5a23d88cee22e9045d43739bfa09b6b4523
Merge "Replacement mox by mock"