Allow usage of server variables other than default

[meesvandongen]

Is your feature request related to a problem? Please describe.
I would like to generate a Kong Configuration for multiple servers using the same OpenAPI specification file.

Describe the solution you'd like
OpenAPI allows specifying "server variables" in the servers field of the specification. These are currently set to the default value by openapi-2-kong. I would propose adding a new command line option to the CLI for specifying these server variables.

Describe alternatives you've considered
Writing a script to replace these variables in the generated config, perhaps substituting with another templating system so it doesn't interfere with openapi-2-kong.

Additional context
An example of a server variable in OpenAPI

{
  "servers": [
    {
      "url": "https://{username}.gigantic-server.com:{port}/{basePath}",
      "description": "The production API server",
      "variables": {
        "username": {
          "default": "demo",
          "description": "this value is assigned by the service provider, in this example `gigantic-server.com`"
        },
        "port": {
          "enum": [
            "8443",
            "443"
          ],
          "default": "8443"
        },
        "basePath": {
          "default": "v2"
        }
      }
    }
  ]
}