Replace "cloudServers" with "cloudServersOpenStack".

For getting the account number, get it from the "cloudServersOpenStack"
returned endpoint instead of "cloudServers".  "cloudServers" is the API
endpoint for 1st gen cloud servers, which is no longer available for
use in newer accounts, and the API response is no longer returning that
endpoint.
Use `rsplit` with indexing instead of `os.path.basename`.

Since we’re not dealing with OS paths, use `rsplit` with indexing
instead to get the last path in the URL.
Merge pull request #41 from lemuelf/dropped_cloudServers_endpoint_fix

Thanks!