Allow to control Bamboo SSH proxy server port

Bamboo SSH proxy server, which is launched both on Bamboo machine and on build agent machines, always binds to a random port. The port is reset on Bamboo restart.

It would be nice to have the ability to control that port - either to provide a specific port to bind or to provide a range of ports (or any form of restriction/control). A possible solution might involve using a system property.