(SERVER-87) Add upgrade behavior to pre_suite.

When PUPPETSERVER_INSTALL_MODE=upgrade is set in the environment from which
beaker is run, the pre_suite will first install the latest version of puppet and
puppetserver from the official puppetlabs release repository, run the
initialize_ssl, procedure, then install puppetserver from the development
repository.

Note that initialize_ssl will only happen once--immediately after the initial
install. There will be no initialize_ssl after the upgrade install.
This isn't related to this PR, is it?  It's basically just a merge-up from stable?