librenms: discovery fixes

- Add snmp-mibs-downloader to fetch proprietary MIBs.
- Explicitly set the autodiscovery modules, and disable SNMP scan.
- Switch from poller.php to pollerwrapper.py, per the manual.
- Add html_dir config option to restore icons

Change-Id: Ibbcc781093e04b133f80f712a56a4b1b0071e002