Fix missing metadata in messages

[adybbroe]

We were missing variant in output messages. This is now fixed.
Also all metadata (except description) in the hook is now being added as a dict, like this example (pps_hooks.yaml):

pps_hook:
    post_hook: !!python/object:nwcsafpps_runner.pps_posttroll_hook.PPSMessage
      description: "This is a pps post hook for PostTroll messaging"
      metadata:
        posttroll_topic: "PPSv2018"
        station: "norrkoping"
        output_format: "CF"
        level: "2"
        variant: DR

This means it is easier to add new metadata to the message if needed.

Also fixing a couple of bugs from previous PR #13

[TAlonglong]

I would have writte if attr not in self.metadata: but I guess both do the same

[adybbroe]

Ah, yeah, right @TAlonglong , looks a bit awkward now that you say it!

[adybbroe]

I was too late seeing this - merged already. Sorry! I will fix next time if I remember!

[TAlonglong]

LGTM