NG: Allow for subpath

[dmp42]

The registry should not assume it's running at the root of the server.

eg: both https://myregistry/v2/ and https://myregistry/something/v2/ should be possible.

The typical scenario when this is happening is a registry being run through a corporate portal that would "mount" the registry backend on a subpath.

This remark is valid as well for client code to be written.

See #284 for historical reference.

[dmp42]

cc @stevvooe

[stevvooe]

Superseded by distribution/distribution#29.