xenapi: Add configurable BitTorrent URL fetcher

Add configurable torrent URL fetcher for sourcing image torrents.

The code uses entry points to find a "driver" style function.  If no
driver is supplied, the torrent file will continue to be found at:

CONF.xenapi_torrent_base_url + "/" + image_id + ".torrent"

DocImpact

Change-Id: Ib2b90c32a5b2c20dd1863058cf1a523672cf2fb3
Merge "xenapi: Add configurable BitTorrent URL fetcher"