[JAVA][library: vertx] Allow subclass override of http server port for testing

[ccozzolino]

Description

For testing, we need the ability to override the http server port without having to generate a custom testing spec with an overridden host value.

Swagger-codegen version

All versions with vertx async & rx server support

Swagger declaration file content or url

Command line used for generation

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

Modify the MainApiVerticle.mustache template to allow a subclass to override the serverPort value assigned by the spec.