OpenStack service: add AuthenticationClient.
Configure using properties relevant to OpenStack.

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>
test_client.py: cleanups to S3 config tests.

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>
test_client.py: add OpenStack config tests.

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>