Add alerting

• Add Blackbox exporter to easily export metrics about health of web services and DNS servers
• Add vmalert to throw alerts when something is wrong, with rich metadata (description, link to Grafana dashboard, criticity, etc)
• Add alertmanager to process throwed alerts and send them to Mattermost with nice formatting

merged

mentioned in commit a8cefb09

mentioned in commit picasoft/projets/services/monitoring@bc70f260