Osquerybeat: Fix osquery logger plugin severy levels mapping

[aleksmaus]

What does this PR do?

Fixes the osquery logger plugin log severity level mapping. For example 0 severity was mapped to error and was logged as "error" in osquerybeat logs instead of "info" level.

Why is it important?

Log level correctness

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas

Logs

Osquerybeat log before the change, showing "error" level log:

 {"log.level":"error","@timestamp":"2021-09-07T21:36:56.248-0400","log.logger":"osquerybeat","log.origin":{"file.name":"beater/logger_plugin.go","file.line":69},"message":"Calling configure for logger osq_logger","service.name":    "osquerybeat","ctx":"logger","osquery.log_type":"status","osquery.severity":0,"osquery.filename":"config.cpp","osquery.line":891,"osquery.cal_time":"Wed Sep  8 01:36:53 2021 UTC","osquery.time":1631065013,"ecs.version":"1.6.0"}

Osquerybeat log after the change, showing the correct "info" level log:

{"log.level":"info","@timestamp":"2021-09-07T22:05:24.843-0400","log.logger":"osquerybeat","log.origin":{"file.name":"beater/logger_plugin.go","file.line":69},"message":"Event publisher not enabled: openbsm: Publisher disabled via configuration","service.name":"osquerybeat","ctx":"logger","osquery.log_type":"status","osquery.severity":0,"osquery.filename":"eventfactory.cpp","osquery.line":156,"osquery.cal_time":"Wed Sep  8 02:05:24 2021 UTC","osquery.time":1631066724,"ecs.version":"1.6.0"} 

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• Start Time: 2021-09-08T02:16:38.938+0000

• Duration: 54 min 26 sec

• Commit: 9186aec

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