Add support for FOG_MOCK in tests
Clarify explanation of how to set up fog mock mode

We are adding the test data to [our shared setup](https://github.gds/gds/vcloud-tools-testing-config/blob/master/vcloud_tools_testing_config.yaml) so this detail is for external users.
Merge pull request #85 from gds-operations/add_mock_envvar_for_tests

Add support for FOG_MOCK in tests