Expose the service catalog

It can be handy to see the whole service catalog, particularly when you don't find the endpoint you were expecting.

This is a more of a for-discussion pull request.  Things of which I am not sure:

* We could instead have one method, which takes a (possibly empty) ApiCriteria and returns all the matching endpoints.

* Should we wrap the ServiceCatalog in an interface?  I think Identity V3 breaks it, so we probably should.

* Is this an Openstack-API binding or a JClouds-style multi-API binding?  If the latter, then we definitely need an interface!