Upgrade Puppet agents to Puppet 4

Upstream doc: https://docs.puppet.com/puppet/4.5/reference/upgrade_major_agent.html

Note that the Puppet 4 master in Stretch is supposed to be able to handle 3.8.x agents. So we can do that incrementally.

Parent Task: #11834 (closed)

Related issues

• Blocks #13284

Original created by @intrigeri on 11838 (Redmine)