Warn or error on unrecognized keys in cloud-config.yml

I had a cloud-config.yml that looked like this:

    #cloud-config
 
    units:
      - name: static.network
        content: |
    [...]

The other stanzas were working just fine, but `units` was being silently ignored. Of course, the correct key was actually `coreos.units` -- but the behavior of silently ignoring the unknown `units` key, instead of throwing an error, made this unnecessarily hard to figure out.

I suggest that if any keys are laying around in cloud-config.yml that shouldn't be, cloud-init should at least warn that it's going to ignore them. (It could also just error out entirely, though this would probably harm reuse of cloud-config.yml files generated by foreign stacks.)