Raise minimum Puppet version to 4.4
Currently there's still a lot of code to support Puppet 3, but that's EOL. There's also some code for early 4.x versions that nobody should be using anymore. 4.4+ doesn't need any special version handling so that'll be the minimum new version.
Refs #26591 - Stub Rubyipmi.is_provider_installed?
With the removal of the Puppet gem, it looks like /sbin is no longer
guaranteed to be in PATH. Since these tests shouldn't care about that,
we can pretend by stubbing.
Fixes #26591 - Remove puppet_proxy_legacy (Puppet 3)
This drops legacy Puppet support and relies on the Puppet API to
retrieve classes. This means at least Puppet 4.4 is required. Since
Puppet 4 also removed puppet kick, that provider is removed as well.