support setting log severity for logs received from in_syslog

[mr-salty]

When fluentd processes logs via the in_syslog input plugin (http://docs.fluentd.org/articles/in_syslog), it converts the tag value by the following pattern:
tag = "#{@tag}.#{facility}.#{priority}"

Our ingestion process currently turns that into a log name. Priority should instead be mapped to the severity level of the log entry.

[palladius]

(Internal bug is b/18731456). Has anyone spare capacity to fix this?

[jkohen]

Seems connected to #51 or even a duplicate