Add Syslog to the list of default Outputs in GL

Syslog deserves to be added as one of the Output types available **_by default_** in GL (and not via a plugin).

## Why?

Syslog is a very popular log mechanism that is supported by most log / SIEM platforms.
Alot of companies rely on Syslog output to ship logs from GL to other systems (long term log repos, analysis systems, etc.).
Therefore I believe it should be an out of the box feature in GL (as opposed to a plugin).

## Your Environment

* Graylog Version: 3.3.8