Add support for PUPPET_REPO=nightly to install nightly Puppet builds

<pre>
[dcleal@cobalt foreman-bats]$ vagrant ssh wheezy -c 'dpkg -l puppet facter'
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                               Version                               Architecture Description
+++-==================================-=====================================-============-=======================================================================================================
ii  facter                             2.2.0.2-1puppetlabs1                  all          Ruby module for collecting simple facts about a host operating system
ii  puppet                             3.6.2.1080-1puppetlabs1               all          Centralized configuration management - agent startup and compatibility scripts
[dcleal@cobalt foreman-bats]$ vagrant ssh el6 -c 'rpm -q puppet facter'
puppet-3.6.2.1080-1.el6.noarch
facter-2.2.0.2-1.el6.x86_64
</pre>

Replaces what I was doing in Jenkins with my old copr builds.

(https://groups.google.com/forum/#!topic/puppet-dev/X1ekljb5x8w announcement)