Add support for cut to ProxyCommand

[russjones]

To use Machine ID, at the moment you must have DNS configured to connect to a host. The ssh_config automatically generated contains the following.

Host *.example.com !proxy.example.com
    Port 3022
    ProxyCommand ssh -F /opt/machine-id/secondary/ssh_config -l %r -p 3023 proxy.example.com -s proxy:%h:%[email protected]

This is expanded to the following when running ssh -F /opt/machine-id/secondary/ssh_config [email protected]:

debug1: Executing proxy command: exec ssh -F /opt/machine-id/secondary/ssh_config -l rjones -p 3023 proxy.example.com -s proxy:server01.example.com:[email protected]

The proxy subsystem will try and match on exact nodename so server01.example.com and always fail to match.

We should update the ProxyCommand to execute a program on disk like cut to manipulate ProxyCommand to work.