(QENG-174) Allow installing a new puppet package on top of the PE tar

Currently, when running the puppet acceptance tests against PE, you
can only test the version of puppet that is shipped with the PE
tarball. This adds another step to the PE pre-suite which will install
an arbitrary version of pe-puppet on top of the installed PE tarball.
I think you'd need to come back to this setup step to understand how to use the env variables anyway, so I think the skip_test message is sufficient.  I'm +1 to merge and will push the green button.