Allow running backend-fsn/hel on EC2

[hellais]

Currently backend-* are run on dedicated hosts, however since we now have a separate clickhouse cluster, the server requirements for running the backend are drammatically reduced, so we can probably afford to run it on an EC2 node instead.

This would also enable us to switch serving api.ooni.io from AWS without getting so impacted by the high exit bandwidth costs (see: #94).

Related to this is also the switch to backend-fsn: #110