Return a random subset of cpus for the cpuset config

def _rand_cpu_str(cpu):
    '''
    Return a random subset of cpus for the cpuset config
    '''
    cpu = int(cpu)
    avail = __salt__['status.nproc']()
    if cpu < avail:
        return '0-{0}'.format(avail)
    to_set = set()
    while len(to_set) < cpu:
        choice = random.randint(0, avail - 1)
        if choice not in to_set:
            to_set.add(six.text_type(choice))
    return ','.join(sorted(to_set))