Allow acceptance tests to run with an API key or alternate authentication endpoint

This change to libargs.go will use RAX_API_KEY as an the APIKey for authentication, and OS_AUTH_URL as an OpenStack authentication endpoint. If neither is set then the current behavior remains the same: the acceptance tests use password authentication against the default endpoint.
This won't work as one would expect.  The provider can be either rackspace-us, rackspace-uk, or an HTTP URL to the authentication tokens endpoint.  There's no need for authURL as far as I can see.