Fix xenapi functions' tests

The tests got outdated, this fix makes the tests pass again.

Change-Id: Iadddfbf34bf79ba455811645e766c2f3d0fcca84
Workaround missing zip snapshot

At the moment, xenserver installation depends on github snapshots.
Unfortunately, git.openstack.org does not have that capability. This
fix includes:

 - Exit with error code, if a download fails
 - create proper urls, even if they are using the git protocol
 - set git base to github - so we are able to do snapshots

Fixes bug: 1259905

Change-Id: I8d0cf8bf8abb16ee0a4b138a6719409c75e7a146