Fluentd config actually used two keywords to named log level:
"log_level" and "severity_label".
This doesn't allow to search and sort ALL logs in Kibana using a unique
field (ex: log_level).
This fix standardize log level keyword naming.

Closes-Bug: #1713410
Change-Id: Ibb2eaf3b6aa21ce6f7945e768e6362c0d40f0507