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

SYNOPSIS
[modified]m42/test/test_zaphod.py: Test _set_nat_ssh_forwarding_port.
[modified]m42/test/zaphodtestutil.py: Get forwarded port info.
[modified]m42/zaphod.py: Implement _set_nat_ssh_forwarding_port

DETAILS
Implemented Zaphod._set_nat_ssh_forwarding_port. This will be used by
createvm to change the forwared port of a machine if it conflicts
with the forwarded port of another machine.