move log config to struct, support disabling stdout for builds

[syphar]

this is triggered by @marcoieni asking us for logging less to syslog for cost reasons.

Especially the build logs inside rustwide::logging::capture are also stored on S3, so we don't need to additionally log them to stdout.

While I'm on it, move the logging config to a config struct.

[GuillaumeGomez]

Thanks!