(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.
Would it be worth specifying in the message what environment variables are expected to upgrade the package?