Load-balance test helpers using haproxy

[FedericoCeratto]

• [x] Test haproxy on backend-hel
• [x] Configure haproxy and dehydrated hooks in ansible
• [x] Test haproxy on ams-pg-test
• [ ] Enable active checks
• [ ] Deploy on backend-fsn

[hellais]

We should move the test helpers to the new terraform based deployments.

See: https://github.com/ooni/backend/issues/807