Support binding to port 0

[BlinkyStitt]

Overview

When running multiple test workers, brownie currently sets the port to port+1. This works most of the time, but only if there is a range of open ports.

More robust, would be to tell each node to bind to port 0 and then let the system decide which open port to use. Then we update brownie's config to use the assigned port and everything should work as normal.

At this time, I don't think we should change the default to 0 (though I think it is a better default). Instead, we should just make pytest -n do this and also support it if its assigned to 0 in network-config.yaml.

Hardhat currently supports binding to 0. Ganache does not (trufflesuite/ganache#2643).