Start only exporters services for dedicated Prometheus installations

When installing Moneo with dedicated Prometheus server, only the exporters services should be started. Right now in `start_moneo_services.sh` there is not an option to skip starting the prometheus exporter.
Although the exporter services can be started manually without using said script, being able to use it in custom deployments will reduce the complexity of being up to date with future changes to the script (e.g. requirements).