Add option to log in json format

[grzesuav]

For use in ELK the log need to be in json format.

[enisoc]

Has anyone had good experience with a Go logging library that supports JSON output?

A couple that I've seen in the wild are:

• https://github.com/sirupsen/logrus
• https://go.uber.org/zap (used by controller-runtime via https://github.com/go-logr/zapr)

Once we pick one, it should be pretty mechanical to go through and switch it out.