fixes #4030 - build via librarian-puppet

Putting this up for comments.

This supports running librarian-puppet in either the build (via the Rakefile) which we'll probably use for Debian packages once they're using the Rakefile, or in the "release" script which is used to build SRPMs.

Both work reasonably well I think, though the release script suffers from a bug when running concurrently as librarian-puppet calls Puppet's module tool, which gets confused when the vardir (~/.puppet/var) is used concurrently on Jenkins.  The Jenkins job is running under RVM like our others, so it installs gems in 'release'.

I got one successful el6 build out of it: http://koji.katello.org/koji/taskinfo?taskID=75163 (http://ci.theforeman.org/job/packaging_build_matrix_rpm/1092/).