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

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

DETAILS
Implemented Zaphod._next_free_nat_ssh_forwarding_ports, that returns
next free port available on the host for assignment for ssh port forwarding
This is used createvm set port forwarding on a newly created VM that
uses NAT.