puppet: install git-client plugin by default

https://wiki.jenkins-ci.org/display/JENKINS/Git+Client+Plugin needs
to be available (at least currently) so the git plugin works.
Depend on subversion-tools (<= 1.6.18dfsg-1) *or* svn2cl

svn2cl is no longer available in the unstable version of
subversion-tools, instead it's provided as separate package
as of subversion-tools 1.7.5-1.

Closes #45
use a more sensible default for DEBEMAIL
puppet: allow DEB_* environment variables in sudo configuration

Related to issue #46
puppet: use wget instead of curl as fallback for public IP retrieval

curl isn't present on Debian's AMI out-of-the-box.
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