[product: zaphod][project: zaphod-v0.3]: Implemented _nat_ssh_forwarding_port

SYNOPSIS
[modified]m42/test/test_zaphod.py: Implemented test__networking_mode
[modified]m42/test/zaphodtestutil.py: Code to set forwarded ssh port.
[modified]m42/zaphod.py: Implemented _nat_ssh_forwarding_port.

DETAILS
Implemented method _nat_ssh_forwarding_port that returns the port number
on the host that forwards ssh traffic to the ssh port on the VM. In
order to test this function, one has to be able to set the port
to something other than the default. The code in zaphodtestutil.py
does just that. A new test has been added to test_zaphod.py.