Improve monitoring of ooniservices

[hellais]

Currently ooniservices are being monitored through prometheus with the job name being used to identify the different environment: https://github.com/ooni/devops/blob/main/ansible/roles/prometheus/templates/prometheus.yml#L93.

We should:

• use a variable to store the list of target hosts
• change this to use a label to identify the environment
• add to monitoring services that are currently not monitored

Once #104 converges we should be generating these lists automatically.