make syslog priority configurable

Set priority or facility + severity. This is a proposed fix for: https://logstash.jira.com/browse/LOGSTASH-1135
Set the priority from configuration or a field (integer). If use_labels is set don't use the priority but parse facility and severity strings from config or field. 
If setting the priority fails fallback to priority 13 according to RFC3164.

I will update the docs if this patch will be accepted.

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