Fix rally.benchmark.scenarios autoload

*) There was cycle reference during autoloading scenarios.
*) Fix typo/bug s/auth_url/uri/

Change-Id: I2473c7aacf73f44abdb1511f859e6d504dd85725

to scenario autoload

Change-Id: I929c7c5d581fc2bc8f9f6c6557fa90247745ece5