Raca::Account#public_endpoint should support non regioned APIs

* some rackspace cloud APIs (like identity and dns) are not regioned, so
  the second argument to Raca::Account#public_endpoint must be optional
* This is required as I prepare to setup per-app users for our rackspace account. I can create the new users via the control panel, but for fine grained control of their access I will need to use the identity API