Clean up mcollective upgrade stories in puppetlabs-puppet_agent

mcollective is removed in puppet 6, and with the removal of puppet 3 support in puppetlabs-puppet_agent, we need to revisit our mcollective tests and ensure we're testing whatever still makes sense in puppet 5.

We also need to update the FOSS upgrade scenarios for puppet6, since foss upgrades will fail since they'll use the services class which will attempt to start mcollective (and fail) in puppet6