Number of worker threads used by the controller can't be configured

[HeGaoYuan]

As the following log shows, the default number of worker threads used by controller is 1. This value is too small for production environment because it will cause extremely delay of processing Job if there are many Jobs at the same time.

And I found this can't be configured.

1.6717679978118246e+09  INFO    Starting workers        {"controller": "pytorchjob-controller", "worker count": 1}
1.6717679978129413e+09  INFO    Starting workers        {"controller": "tfjob-controller", "worker count": 1}
1.6717679978130171e+09  INFO    Starting workers        {"controller": "mpijob-controller", "worker count": 1}

Referring to point4 of #1703