change snmp_enabled default from true to false so devices like
resolver1 that only need ping/dns don't trigger unnecessary snmp
discovery. skip sending snmp_version/snmp_port to the api when
snmp_enabled is false.
- Make site_id optional, add organization_id field
- Devices can be created without sites (directly in organization)
- Add SNMPv3 authentication and privacy fields
- Update documentation with new examples
