Deprecate Puppet WEBrick and Rack servers

The Puppet WEBrick and Rack master servesr should be deprecated, as we plan to eventually remove them in favor of Puppet Server.

From kylo:

I'd like to add a deprecation notice at the same time, since the plan to deprecate, and later drop, webrick is in part what has motivated the changes that lead to this issue.

This refers to PUP-4394, wherein the puppet-agent package doesn't create the puppet user by default so WEBrick fails to start unless the user is manually created.

Risk Assessment: low
Probability: low
Impact: low
Test Layer: unit