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
display the current hostname in build output
Merge pull request #6 from conversation/account-should-support-non-regioned-apis

Raca::Account#public_endpoint should support non regioned APIs
Raca::Account#public_endpoint should handle 'identity' as a special case

* the identity API isn't included in the service catalog, it's the one
  API endpoint that is hard coded
* Until now it was only used in the Account class, but we're about to
  start using it for user management
WIP experimenting with retrieving users from the identity API