puppet: do not install xunit plugin by default

Junit is available out-of-the-box in Jenkins and our configuration
actually uses that, so there's no point in installing the xunit
plugin by default.

Verified on fresh Ubuntu 12.04 EC2 system.

Thanks: Sylvestre Ledru <sylvestre.ledru@scilab-enterprises.com>
for bringing this up in https://github.com/mika/jenkins-debian-glue.org/pull/4