Generate Prometheus configuration from config-svc

When we start using the Couchbase Server 7.x /metrics endpoint, Prometheus will need basic authentication details for it, so have config service generate prometheus.yml with the details.

The best approach (as captured in CMOS-104) is likely to use Go templates in some way.

Note that this creates a startup dependency - config-svc must have set up the Prometheus config before Prometheus attempts to load it.