Adding ability to run XmlLint for each API call.

This will not run in Tox normally, but makes that idea possible.
See the following for more details:
https://wiki.openstack.org/wiki/Reddwarf-Test-With-XmlLint

Change-Id: Ifc675b6955b1e5d5e32c98968f1444de9dffe14c
implements: blueprint xmllint-verification
Merge "Adding ability to run XmlLint for each API call."