(QA-845) Allow PE upgrade version to be specified in the config

Previously, when doing an upgrade beaker could only get a version by
reading the LATEST file. This makes it difficult to track exactly
which version is going to be tested in a given test run. By allowing
the version to be specified in the config file, we can make sure
beaker installs precisely the PE version that we care about for
testing.
Merge pull request #154 from branan/programmable_upgrades

(QA-845) Allow PE upgrade version to be specified in the config