Ensure authenticate never re-auths.

Other API functions will support re-auth as a matter of course.
If an auth token expires, we need to re-authenticate to acquire a new
token.  If re-authentication were itself to attempt re-auth, we
would end up in an endless loop.

If after authenticating gophercloud receives a 401 Unauthorized
response, then we must assume that the provided credentials are
incorrect.
4 files changed