xenapi: stop hang during glance download

It seems there are cases when the server gets stuck in a building state
because the download from glance fails, but the nova-compute doesn't
find out.

After some code inspect, a likely cause seems to related to having no
socket timeout inside urllib2 when we download the image from glance. A
workaround for this issue is to configure the default socket timeout as
described here:
http://docs.python.org/3.1/howto/urllib2.html#sockets-and-layers

Fixes bug 1251650
Change-Id: I8b34d95d3524f3825b28e61cbbb59290653d3711
Merge "xenapi: stop hang during glance download"