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

SYNOPSIS
[modified]m42/zaphod.py: Implemented execute_shell_command.
[modified]m42/test/test_zaphod.py: Test execute_shell_command for NATed VMs.

DETAILS
execute_shell_command executes shell command on the VM and returns the
result. This is used for systems management of VMs. The
execute_shell_command relies on _login_parameters to get login
information. This function currently only works for NATed machines.
Also, it maybe better to use something like fabric to execute this
stuff.