Add oslo.rootwrap to devstack gate

oslo.rootwrap recently graduated but was not made part of the
devstack-gate. This change is part of a series of changes affecting
devstack-gate, config and devstack which will collectively fix this:

https://review.openstack.org/#/q/status:open+topic:rootwrap-gate,n,z

This should probably be merged once the config and devstack-gate changes
are in, so that it can be self-testing.

Change-Id: I7b1332c8004845a0dd76e27d871370d41d4524ac
Merge "Add oslo.rootwrap to devstack gate"
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The tests got outdated, this fix makes the tests pass again.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