use env var to filter services with the same host in the Public Base URL

The request is to have a new environment variable that filters services with the same Public Base URL. For example:

Service 1:
staging: https://mygateway-test-staging.com
production: https://mygateway-test.com
Service 2:
staging: https://mygateway-qa-staging.com
production: https://mygateway-qa.com
Service 3:
staging: https://mygateway-qa-staging.com
production: https://mygateway-qa.com
Service 4:
staging: https://mygateway-prod-staging.com
production: https://mygateway-prod.com

Service 2 and 3 uses the same gateway. If the env var is set to: https://mygateway-qa.com, the gateway would only download the configuration of the services 2 and 3 for production environment.

Currently the services can be filtered with APICAST_SERVICES_LIST env var, but you need to provide the service_id and set the list in the env var. When there is a large number of services setting the hostname would make it easier.

The example provides different Public Base URL for staging and production, but It can be the case that staging and production have the same URL.