Make the service definition argument list to be more flexible

Currently, the custom arguments given when starting a service port are always appended at the end of the predefined argument list with variable interpolation.This means that we now cover the service launch commands like this:`["some_service", "-a", "1", "-b", "2", ...]`

Let's allow inserting the custom arguments in the middle, to cover cases like:`["some_service", "-a", "1", "-b", "2", ..., "run"]`where the client-provided custom arguments should be put at the place of `...`.

We need to define the marker string (maybe it could be just `"..."` and/or `"{:custom_args}"`) and perform removal of it or replacement of it depending whether the custom arguments are given or not.

JIRA Issue: BA-295