ChooseVersion accounts for missing /s.
Right, v2 actually needs those vars.
Update the identity v2 acceptance test.
Lots of URL normalization.

So it doesn't matter if you have a trailing / or not.
Ookay, last reshuffle.

 * openstack.NewClient() normalizes the identity endpoint with a trailing slash, and sets base and endpoint.
 * utils.ChooseVersion() checks suffixes first to short-circuit actual version calls.
 * gophercloud.ProviderClient distinguishes between the root of all identity services (IdentityBase)
   and the endpoint of the requested auth service (IdentityEndpoint).
Explicitly auth against v3 in v3 tests.

Skip v3 tests altogether if no USERID is set.