Support non public cloud CNS domains

DNS/CNS information is [hard-coded for Triton Public cloud](https://github.com/autopilotpattern/prometheus/blob/master/etc/prometheus.yml.ctmpl#L51-L52):
```
        dns_suffix: 'cmon.{{env "TRITON_DC"}}.triton.zone'
        endpoint: 'cmon.{{env "TRITON_DC"}}.triton.zone'
```

Those should have an env var that users can change to make it work in other clouds with Container Monitor.