Allow setting server path in channel.ini

This is obviously a wishlist item.

It's (to my understanding) impossible to specify in the system-image client config that a system-image server has a root directory that is not "/" on some web server.

For small-scale community ports, using google drive (or possibly ubuntu one if it supports https and direct linking?) as a host for the files created by system-image server appears to be a simple way to share porting results. That's only possible if we could set something like "base_path: /host/0Bxs[..]kE" in the channel.ini file so system-image client would check googledrive.com/host/.../stable/vision/index.json for updates instead of googledrive.com/stable/vision/index.json like the current implementation does when extending the base property.

(Another issue is that system-image client attempts to download updates using http://, which is not supported by gdrive, and the 301 Moved Permanently to https:// is ignored by u-d-m so every "successfully" downloaded update does not actually contain .tar.xz file contents, but a HTML page saying "This document has moved". Creating a u-d-m bug for that.)

I came up with a hack to use a "base_path" param and force all downloads to use https. This seems to work with google drive for the time being. Once I feel confident this port works good enough I can already use gdrive to share it, so this is even a very low priority for myself, but maybe it helps attract more community porters. My hacks don't play well with the "urljoin" python function that cuts of the path in some places and requires ugly changes so I'm not attaching the branch because it's not useful for a proper solution to this wishlist item. However if a porter is interested I'd be happy to share it.