Use json as default log format

All CTA logging efforts have been directed toward json log format for quite some time now.

Yet a CTA newcomer starting cta-taped orcta-frontend with no specific log option will use key-value by default for logs. He is therefore likely to waste plenty of efforts on the wrong log format.

Maybe it is time to use json as the default log format for all the components that support it and allow users to configure the old key-value if they really want to stay with it.

We also need to give it a proper name: current values are default/json

We should move to something like json/key-value with json as default. Terms like default or legacy is confusing and does not describe the log format: especially if json is the default, default... would not mean much.

Another possibility would be to drop the legacy log format entirely.