Add log level to log lines

[dneray]

We are trying to parse and forward mosquitto logs into a searchable database. It would be very useful if we could include the log level as part of the message.

It might be useful to generalize this in the configuration to being able to customize the log messages i.e.
log_format %l %t %m

where %l is the level, %t is the timestamp and %m is the log message.