Make `json` logs the default and add `json-pretty`

The `json-pretty` log format flips on the “pretty” flag for logrus
which gives multi-line formatted json log output.

This also changes to make the `json` format to be the default as it
is likely to be what deployers want when running with `—server`.

Fixes: #1202
Signed-off-by: Patrick East <east.patrick@gmail.com>