Add public user to serverless test

[pquentin]

Rally does not currently support serverless public users, which is why we use pytest.xfail (which was simpler to use than pytest.mark.xfail due to the parametrization). Usefully, it does print the error, which is currently:

elasticsearch.NotFoundError: NotFoundError(404, 'Request for uri [/_cluster/health?wait_for_nodes=%3E%3D1] with method [GET] exists but is not available when running in serverless mode', 'Request for uri [/_cluster/health?wait_for_nodes=%3E%3D1] with method [GET] exists but is not available when running in serverless mode')

I'd suggest reviewing the two commits separately as the first reorganizes the code.

[gbanasiak]

Thank you. LGTM.