Handle variables in server declaration

[jmini]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee: @OpenAPITools/generator-core-team

Description of the PR

Fixes #592.

With OAS3 it is possible to have variable in the server declaration:

openapi: 3.0.0
servers:
  - url: 'http://localhost:{port}/sample'
    description: 'Development server running on localhost'
    variables:
      port:
        default: '8080'
        description: port number on which the server is running

[ackintosh]

Looks good to me. ✨