URL prioritization backend: test

[FedericoCeratto]

Next step for #361
TODO:

• configure local Nginx to serve files based on query
• implement fallback to static files if the backend stops updating the dynamic files
• switch traffic from orchestration nginx

[hellais]

This is the nginx config for the probe-services: https://github.com/ooni/sysadmin/blob/master/ansible/roles/probe-services/templates/probe-services-nginx

[FedericoCeratto]

The nginx on ams-ps2 is receiving GETs for /api/v1/test-list/urls?category_codes=....&limit=17&probe_cc=... that should be redirected to the prio service. ooni/pipeline#312