Add config file option to control journal probe log level

[phmccarty]

By default, the journal probe will capture systemd journal log messages with log levels 0-4 (error conditions), but it will be helpful to have more fine-grained control over the log level threshold. For example, only sending telemetry for the two highest log levels, or also including LOG_WARNING.

[phmccarty]

The log level message capturing only happens when privacy filters are disabled, but this is still a nice feature to support.