XenAPI: adjust bittorrent settings

This commit addresses some optimizations when using libtorrent to
download.  The settings are documented here:
http://www.rasterbar.com/products/libtorrent/reference-Settings.html

I'm proposing to add these with dom0 environment overrides in case
any other adjustments are needed.  The changes effectively double the
download rate from the default settings.  It also allows unbalanced
download/upload which is preferred in openstack environments.

No unit tests were added as plugins are not currently unit tested.

Closes-bug: #1303993

Change-Id: I503a6a521c196e4ad58e0a16aba2988f66a9cdca
Merge "XenAPI: adjust bittorrent settings"