[BUG] Server Variables not working with Multiple Servers

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

If you specify multiple servers with server variables in your yaml file, their URLs are not properly made available at template render time. Instead, it shows their URLs with the non-replaced variable names.

openapi-generator version

Am using master @ commit 77f2a25c3d

OpenAPI declaration file content or url

servers:
  - url: http://127.0.0.1.nip.io:{listenPort}/{commandLineServerArg}
    description: Debug Localhost
    variables:
      listenPort:
        default: '5000'
        description: Port on which server listens
  - url: https://{username}.gigantic-server.com:{port}/{basePath}
    description: The production API server
    variables:
      username:
        # note! no enum here means it is an open value
        default: demo
        description: this value is assigned by the service provider, in this example `gigantic-server.com`
      port:
        enum:
          - '8443'
          - '443'
        default: '8443'
      basePath:
        # open meaning there is the opportunity to use special base paths as assigned by the provider, default is `v2`
        default: v5/

Command line used for generation

./run-in-docker.sh generate -i /gen/spec.yaml -g python -o /gen/python/ --server-variables listenPort=3456,commandLineServerArg=woof

Steps to reproduce

Run command above on supplied yaml. Will see in output file configuration.py @ about line 338 that servers do not have template variables replaced.

Related issues/PRs

#6337

Suggest a fix

One potential fix is in the pull request noted above. PHP templates deal with this by rendering all of the server information into their configuration template/file, ignoring serverVariables specified on command line to openapi-generator (--serverVariables).

Best is probably to have all templates individually handle. Command line serverVariables should be added to the bundle that is supplied to templates, and individual language templates should fill in such variables at generator run-time.

For variables specified in the yaml/json file, they can remain un-substituted at generator run-time, and instead filled in with logic in the rendered templates.

Wdyt?

Reproduce the supplied OpenAPI declaration and generation command, then inspect the generated Python configuration.py around line 338. Compare the behavior with related issue/PR #6337 and the PHP template handling described here. Done means server URLs are rendered with the appropriate variable values rather than unresolved variable names.