Shell out to an external command and return the output or the empty
    string in case of error.

def shell_out(cmd, timeout=30, chroot=None, runat=None):
    """Shell out to an external command and return the output or the empty
    string in case of error.
    """
    return sos_get_command_output(cmd, timeout=timeout,
                                  chroot=chroot, chdir=runat)['output']