Adding support to set port externally with system property

[cardil]

Unfortunately Spark doesn't support setting port from system property. It's sometimes nessesery to give port for example in Heroku https://devcenter.heroku.com/articles/java-faq#what-constraints-should-i-be-aware-of-when-developing-applications-on-heroku:

The web process must listen on one and only one port. The port must be the one specified in the $PORT variable. If your process listens on other ports, it will be shut down by Heroku.

In Spring Boot you can give port from command line like this:

java -Dserver.port=$PORT -jar spring-app-1.0.0.jar

In WildFly Swarm you also can give port like this:

java -Dswarm.http.port=$PORT -jar swarm-app-1.0.0.jar

I'm proposing to solve this by using the same property name as in Spring Boot server.port for easier migration between those two.

This PR will also makes posible my feature for Gasper: wavesoftware/java-gasper#1

I tested those changes with test application.

[jkwatson]

I've run spark apps on heroku a lot, and it's never been an issue to grab the PORT environment variable as a part of the Main method that you start your app with, and initialize Spark with it.

[cardil]

@jkwatson You apps are using Heroku convention then. I'ts vendor locking. I'm proposing to use library solution, like Spring and Swarm have done.

[tipsy]

@cardil Can you update this?

[tipsy]

I'll close this then. Feel free to re-open if you want this added after all.