Make host variable in Postman collection more granular

Currently, the `host` variable for the Postman collection contains the entire URL composed of `host` and `service region`, whereas `host` can be `api`, `import`, or `connect`.
This makes it hard to use the same Postman environment for the different API collections we have for HTTP API, Import API, and Connect.

Would be good to have more granular variables in the Postman collections allowing for more flexibility regarding combination of hosts and service regions.