mgr/dashboard: automated tests for Prometheus configuration
Currently, there's no mechanism in place that prevents to use breaking changes or syntax errors in the Prometheus configuration, which also includes the Prometheus alerts.
By utilizing `promtool`, we can have a very cheap but working test to ensure that compatibility or syntax isn't broken.
The scope of this tool is limited to Prometheus' configuration and will not prevent any breaking changes in Grafana dashboards.
promtool for nautilus and octopus should be used to ensure compatiblity for Prometheus v2.7.2. Pacific may be tested by the most recent promtool.