Test the ecs-discovery.py script in the CI

[LDiazN]

The ecs-discovery.py script introduced in #186 allows ECS tasks discovery and it's called periodically with a cronjob in the monitoring host to collect tasks details (EC2 host with private IP, running port) that are used for file-based discovery of scrape targets in Prometheus.

This script is quite long and a bit complex, so it's a good idea to add some testing for it, see #186 (comment)

As a result, the CI should be able to check if this script is able to write the right json and that it has the right format for Prometheus