Add trace log level

[squaremo]

In fluxcd/image-automation-controller#190 I added some tracing output to the image automation controller. Since that controller uses pkg/runtime/logging, and that doesn't know about --log-level=trace, I had to put the tracing in the debug log level. It would be good to recognise the trace level, though.

• parse --log-level=trace
• add a const for the log level so it can be referred to in log.V(CONST).Info(...)