Add --log-format server flag to configure output format

[phillipj]

These changes allows the logger format to be specified when starting the
server, either to text (default) or json.

I'm also interested in adding support for the fluentd format, though that would mean adding a 3rd party dependency: https://github.com/joonix/log.

Any thoughts about the changes as is and/or possible new dependency?

Refs #52

[nckturner]

This is fine with me as is, but I'd prefer not to add the other dependency you mentioned. (Sorry about the long wait for feedback). You'll need to sign the linuxfoundation CLA now though, to get this merged since we moved to kubernetes-sigs.

[phillipj]

Thanks for reviewing! You not wanting to add a new dependency for this is totally understandable, no problem for me dropping that idea.

Pretty sure I got that CLA signed now, at least that's what I interpret from the verification email titled You just signed CNCF Individual Contributor License Agreement. The inline status in this PR hasn't updated though, but that might take a little while? 🤷‍♀️

[nckturner]

/lgtm

[nckturner]

Can you rebase @phillipj?

[phillipj]

Sure thing, rebased and force pushed -- conflicts were trivial enough that a 3-way merge did the trick.

[nckturner]

/lgtm