It could be easily confused by paths containing component keywords as
part of e.g. the image name. Fix this by:

- splitting up by path components
- look for keywords from the right

"repository" has been present since V1, but was not stored. Do this to
prepare for the "serve_content" feature which will need it.

Implement option to send Docker files directly from local paths
instead of creating a HTTP redirects. The local paths can be created by
a Pulp Docker web distributor or by a rsync distributor.

Example crane.conf:

```
[general]
debug: true
data_dir: /var/www/pub/docker/
endpoint: localhost:5000

[serve_content]
enable: true
```

closes #3857
https://pulp.plan.io/issues/3857