Enforce fixture events order

[balanza]

Description

Change the dump_scenario utility so that the insertion order of the discovery events is preserved in the exported fixture data.

Furthermore, the scenario healthy-27-node-SAP-cluster has been re-generated so that its events are ordered.

How has data been re-generated?

# start a fresh instance of the database 
docker-compose down -v postgres  && docker-compose up -d postgres
# replay fixture events events 
photofinish run healthy-27-node-SAP-cluster
# dump stored and discarded events
rm test/fixtures/scenarios/healthy-27-node-SAP-cluster
mix eval "Trento.Release.dump_scenario([\"healthy-27-node-SAP-cluster\", \"-p\", \"test/fixtures/scenarios\", \"-d\", \"100\"])"
# append discarded_discovery_events.json to fixture events
(manual)

Repeat the algorithm until the discarded_discovery_events is empty after a run; that means all events have been correctly ingested by Trento Web and thus are in the correct order.

[nelsonkopliku]

LGTM

[balanza]

This PR has been made upon assumptions that might be invalid and need further reasoning.

More specifically, exporting commands in order is indeed correct but leads to too many files; the actual implementation just keeps the last discovery per host.