Improve the parsing of the SSH proxy_command

[giovannipizzi]

When reading the proxy_command from YAML file in verdi computer configure ssh, and I think also when reading it from command line parameters, the string is not cleaned up from %h, %p and/or from >/dev/null bits.

This logic is present only in the function _get_proxy_command_suggestion_string(cl�s, computer) , that is used to provide the default value by reading from the ~/.ssh/config file; it relies on paramiko itself changing %h and %p when reading the config file, and has some logic for removing the redirections.

I think the logic should be extracted to a function, possibly we need to implement the logic for %h and %p if needed, and then call this logic not only to provide a default, but also to validate and change the input of the user.

Otherwise, a second option: we don't change what the user passes (might be a better idea), but we need to document what we expect as a value (i.e. replace %h with hostname, don't put redirection, ...) and validate the user input, complaining with an informative message if %h or %p are found, or a redirection character > is found.