Support for dynamically allocated stop port

[eskatos]

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We run integration tests on CI and using fixed ports is not an option because other services/tests may hold them.

Allocating a port up-front is not an option either, it could be taken by something else between the allocation time and it's effective usage.

Setting httpPort and httpsPort to 0 works, they get dynamically allocated and parsing info logs or tomcat logs allow to know which ports were choosen.

But it doesn't work for stopPort, here's the exception:

Caused by: java.lang.IllegalStateException: Bad stop port
	at com.bmuschko.gradle.tomcat.internal.ShutdownMonitor.<init>(ShutdownMonitor.groovy:35)
	at com.bmuschko.gradle.tomcat.tasks.AbstractTomcatRun.startTomcat(AbstractTomcatRun.groovy:363)
	... 101 more

It would be nice if one could set stopPort to 0 so it's dynamically allocated.

I guess the complexity will be in getting tha dynamically allocated port and assign it to the corresponding stop task.