[identity] Add tests for list_tenants (public)
[identity] Add failing test that reproduces bug #65

This test shows that list_tenants will fail with a cryptic message
when the user authenticates against keystone without a tenant name
which results in an unscoped token that doesn't have a service
catalog in place.
[common] Add ability for base request to use base_url if provided.
[core] Add error check to Service for when token is not scoped.
[core] Add ability to specify a base_url when Service#request is called

The test is created in a specific request definition (list_tenants).
Merge pull request #67 from relaxdiego/fix/65

This merge adds the ability for the client to specify a base_url for 
the request. This is useful in situations where the session in use 
is based on a default token which does not have any service catalog.