Cleanup syslog fields

[mrdavidlaing]

This PR:

1. Ensures that @shipper.host is always populated with the address of the machine sending the data.
2. When logstash.metadata_level == DEBUG, adds
   • @ingestor.timestamp
   • @ingestor.service
   • @ingestor.job
3. Removes host, received_at, received_from (since these are captured by @shipper.host and @ingestor.timestamp`
4. Ensures @shipper, @source and @ingestor are objects (rather than strings with periods in them; which is not allowed in ES 2.0)
5. Cleans up the fail/syslog_standard/* error tags
6. Retain the syslog_hostname and syslog_timestamp fields

[mrdavidlaing]

Related - 3bf496a57dd5c68d7cd453703ade6793cd62a157

When the syslog message gives us additional information about the original message host (via [syslog_sd_params][host]), treat the syslog_hostname as the @shipper.host and the @source.host as [syslog_sd_params][host]

Specifically:

• [@shipper][host] => %{[syslog_hostname]}
• [@source][host] => %{[syslog_sd_params][host]}