Upgrade Puppet master and clients to 3.8

Originally created by @intrigeri on #11835 (Redmine)

That’s one recommended step towards migrating to Puppet 4, e.g. so that agents can connect to a v4 puppetmaster: https://docs.puppet.com/puppet/4.5/reference/upgrade_major_pre.html#update-to-the-latest-puppet-server-11x-puppet-38x-and-puppetdb-23x.

Parent Task: tails/sysadmin#11834

Related issues

• Blocks tails/sysadmin#11836
• Blocks tails/sysadmin#11833