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