Merge branch 'noc-1515' into 'master'

noc/noc#1515 Add Sensor ETL model.

See merge request !5371

(cherry picked from commit 1ae1b6cb)

ee285b76 Add Sensor ETL model.
60b69dc3 Set workflow_event_model to Sensor.
03da772c Add remote_system, remote_id to Sensor.
5ccb67be Fix typo.
21029c9b Cleanup MeasurementUnits on sensor loader.