allow 'fms_id' as a synonym for the attribute 'external_id' in incoming reports

[davewhiteland]

Since we're anticipating some special handing within FMS for FMS-endpoint (in order to send the FMS report's id with each request) there's a case for using fms_id to avoid nameclashes with external_id.

Actually this is already configurable so this is as simple as changing the default, once we've updated FMS to handle it.

It's unlikely to matter, but may as well head this off at the pass before it happens.