Bye bye tenant field

Since the identity endpoints don't actually require the `tenantName` (and actually will return it on a call), let's just take it out. Otherwise, we're requiring more than the raw calls make. This also breaks other packages that assumed the `tenantName` wasn't necessary.

[Other Rackspace documentation](http://docs.rackspace.com/servers/api/v2/cs-devguide/content/curl_auth.html) does so without the tenant id as well.

The only reason I can see for leaving the `tenantName` in is if the identity service changes at Rackspace.