Canonical push/pull URLs

This merge request fixes issues #112 (closed). There is now a single 'url' specified for any artifact cache. If that's an SSH URL, pulling will still work fine.

There is no attempt at backwards compatibility for existing projects or artifact caches: if merged, everything will break because 'push-url' and 'pull-url' are no longer valid fields. Also all artifact caches will need updating in order for new versions of BuildStream to be able to access them over SSH.

We need to think on how to smooth the transition; however I think it makes sense to work on issue #85 (closed) (pulling from multiple artifact caches) before implementing any backwards compatibility code. I'll look at that next.