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

SYNOPSIS
[modified]m42/test/test_zaphod.py: Test _used_nat_ssh_forwarding_ports.
[modified]m42/zaphod.py: Implement _used_nat_ssh_forwarding_ports.

DETAILS
Implemented Zaphod._used_nat_ssh_forwarding_ports, that returns
a list of ports on the host that have already been assigned to
forward traffic to existing VMs' ssh ports. This will be used by
createvm to figure out an available port for NAT forwarding of
ssh traffic.