Add passwd hash sourced from Foreman

Done as a full users hash, as a class param. This means:

a) We decouple the users from the manifests (apart from keys)
b) We can override the whole hash per-host if we need to add people to single hosts
c) We can add more things to the hash later (ssh keys, shells, etc)

Should remove the fallback code once we're happy it's working.