Output JSON by default.

[metalmatze]

As discussed in some comments over at the Prometheus Operator, we might want to output JSON as default and make YAML a flag which has to be turned on.

prometheus-operator/prometheus-operator#1898 (comment)

[brancz]

My current thought: prometheus rules in yaml, dashboards in json and everything else defaults to yaml. I'm not entirely happy with not streamlining though, so I'll keep thinking about it.