Allow configuration of environment variables for runner services

What does this MR do?

Provides an environment configuration field for runners.docker.services and runners.kubernetes.services that sets/overwrites environment variables for service containers that are defined by the runner configuration.

Why was this MR needed?

We need to be able to provide runner specific variables (originating from puppet) to the services defined via the runner configuration. This seems like the logical place to do that.

What's the best way to test this MR?

Defined a number of service environment variables via the runner and confirm that the service has access (perhaps have a service write the variables to its stdout upon startup and check the docker logs.

What are the relevant issue numbers?

None that I could find.