Return a sort lambda function for the sortedbykey

def _sort_lambda(sortedby='cpu_percent',
                 sortedby_secondary='memory_percent'):
    """Return a sort lambda function for the sortedbykey"""
    ret = None
    if sortedby == 'io_counters':
        ret = _sort_io_counters
    elif sortedby == 'cpu_times':
        ret = _sort_cpu_times
    return ret