Harmonize facility_labels and severity_labels with logstash-output-syslog

[breml]

In logstash-plugins/logstash-output-syslog#34 the question was raised, if the facility_labels and the severity_labels should be harmonised between this plugin and logstash-output-syslog.

In my opinion, both plugins should use the same names for facility and severity labels and it should be therefore possible to have a Logstash configuration like this (detailed configuration of the input and output plugins left out for readability):

input {
  syslog {}
}
output {
  syslog {}
}

Currently this would work, if the numeric value for priority is used. If the labels are used, the output would not be the same as the input was.