Openstack Identity v2 has three distinct ways to authenticate:

1) With username/pwd (credentials), and no tenant specified. This returns a token that is unscoped to a specific tenant.
2) With username/pwd/tenant. This returns a scoped token for the tenant specified.
3) With a tenant and an existing auth token. This returns a scoped token for the tenant specified.

These are specific to OpenStack, not to HP.

In addition, there is a pattern in fog that the auth happens automagically within the ```new``` of the Identity class. There isn't the concept of a specific authentication taking place.

FWIW - I'd prefer they were two distinct operations, but I digress...

If we add specific public auth methods, I believe that would be a deviation from standard fog patterns.