Add parameter for running command via SSH

The SSH protocol supports remote execution of specific commands (rather than the login shell). For example:

$ ssh user1@server1 'command1 | command2'

This support is missing from Guacamole's SSH. Within Guacamole, the SSH connection always goes to the default shell, and there is no way to override this with a specific command at the connection level. Such a configuration parameter should be added.