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
Update links in README

* github to git.openstack.org
* ML openstack to openstack-dev as that is where development discussions
  happen
* nova.openstack.org to docs.openstack.org/developer/nova as that is
  the nova.openstack.org just redirects to there

Change-Id: I884bb2dff3d026618c0bce202f938c7e87bffe04
Merge "Update links in README"
Merge "XenAPI: adjust bittorrent settings"