Add open-source support for MacOS upgrades using puppet_agent

We ship open-source agents for MacOS platforms, and thus should support MacOS upgrades in puppet_agent.

currently, https://github.com/puppetlabs/puppetlabs-puppet_agent/blob/master/manifests/osfamily/darwin.pp#L10L12 stops open-source upgrades from completing on macos platforms.

We need to remove that clause and replace it with open-source sources for packages and settings.