[Review only] Refs #3308 - Auto-create mappings hash from detected plugin classes

Not to be merged until after [puppet-foreman/156](https://github.com/theforeman/puppet-foreman/pull/156) as it contains entries in the answers.yaml for classes which don't exist yet :)

The sed hackery in the Rakefile is annoying though - I might rewrite it to do it properly in Ruby....