Handle variables in server declaration

Created by: 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\.
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• Copied the technical committee: @OpenAPITools/generator-core-team

Description of the PR

Fixes #592 (closed).

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