librenms: add librenms::syslog class

Add a librenms::syslog class which sets up a syslog listener that
pipes to LibreNMS.

This sets up a separate rsyslog instance that receives messages in
syslog (UDP 514) and pipes them to syslog.php.

Using a separate instance seems more complicated at first but provides
certain important benefits:
* The separate instance runs as the librenms user and hence is able to read
  the configuration file without giving access to the whole syslog
  group.
* There's no mixing of system syslog with the remote syslog and no messy
  filtering to avoid logging the local system's logs to LibreNMS.
* The received loglines are only stored in LibreNMS and are not forwarded to
  the rest of the syslog config (local log files, remote syslog servers
  etc.).

While at it, also add some default options to ignore spammy Juniper
lines from the logs.

Change-Id: I7428047f04ae690af807f211218537c8e79628d0