Allow monitoring of external services

[jetzlstorfer]

Currently, the keptn configure monitoring prometheus CLI command creates scrape jobs only for services that are running on the same cluster as the Prometheus installation.
There is currently no way to define the HOST or ENDPOINT of the services I want to configure Prometheus for.

Currently, services are located via svc.cluster.local:

Desired features:
It should be possible to also use keptn configure monitoring prometheus for services that are not running on the same cluster as Prometheus.