howto/puppet: update proposed steps

this updates the puppet control repo proposal to account for the fact
that the repository would track an environment as opposed to /etc/puppet
altogether

it also adds to the proposal renaming hiera -> data, site-modules -> site and
adding "legacy" to the environment modulepath which would contain all
modules which still need to be either published as a public module or
transformed into a profile

note that while I've tried to retain consistency with "step 0" of
publishing our puppet code, I don't think we should maintain this goal,
let alone keep it as the very first step before everything else