nixos/buildbot-master: support reporters, migrate away from status

Since Buildbot 0.9.0, status targets were deprecated and ignored.
There's a very small line on startup explaining that, and status simply
isn't reported. Avoid others the same headaches, and do it right in the
NixOS module.

As there might have been changes in the way reporters are organized, and
configuration might need to be migrated remove the old option, and not
just provide an alias.