Artifact configuration is confusing and fragile, need canonical push/pull urls.

Currently artifact cache configuration is confusing because we need to set the push and pull urls separately.

This leads users to believe that one can set the push and pull URLs to different artifact caches; however in order for BuildStream to know what needs to be pushed and what can be pulled from a given artifact cache; it is essential that BuildStream rely on knowing that the push/pull url refer to the same remote artifact cache.

This is closely related to issue #111 (closed)