xenapi: Make BitTorrent url more flexible

Change xenapi bittorrent plugin to not assume that torrent file
is under CONF.xenapi_torrent_base_url + image_id + ".torrent".

This allows for sources images from arbitrary torrent URLs.

Change-Id: Ica9462a51c6299b5affc861781f3054d18866458
Merge "xenapi: Make BitTorrent url more flexible"