(Maint) Run tests from installed packages

This creates the basic structure for running tests from packages. It
includes a Rakefile for fetching the appropriate artifacts and executing
systest, the setup steps for installing the repos and packages, and a
basic test that makes sure that puppet is installed.

  bundle install
  bundle exec rake test SHA=COMMITISH

where COMMITISH corresponds to the SHA of a package built via the
puppetlabs-packaging job.