Using findAvailableTcpPort is prone to failure when another process
starts using the available port before Undertow starts. This commit
changes UndertowEmbeddedServletContainerFactory to pass the value of
zero down into Undertow where it will ultimately be passed to a
ServerSocket which will then use the underlying OS’s support for binding
to an available port.

Undertow doesn’t provide an API for getting the port(s) on which it’s
listening. Previously, reflection was being used to access the listener
configuration but, when a random port is used, this configuration would
return zero rather than the actual port. The reflective logic has been
updated to look at Undertow’s channels from which the underlying
ServerSocket can be accessed and the local port retrieved

Closes gh-2584