Add /var/log/syslog to Filebeat defaults

[widhalmt]

So far we're sollecting /var/log/messages which is ok for rpm based distros with (r)syslog enabled.

We need 2 changes:

• Add /var/log/syslog to default Filebeat configuration (adding it as a path is totally fine, you don't need an extra "input")
• Add rsyslog and similar tools to requirements