[aks] watchdog execute ssh to node cmd hanging

[YanjieGao]

i test script inner watchdog container.

it is hanging at this funtion [ssh to each node exec docker check cmd]. comment this will not hanging
https://github.com/Microsoft/pai/blob/4f23c079cc4b233a24485af01e5777a8a391ce70/src/watchdog/src/watchdog.py#L400

logs:

i test the cmd directly on that host, not hanging, maybe the python ssh lib lead to this issue?

This is my working branch: https://github.com/Microsoft/pai/blob/yanjga/aks-prometheus/src/watchdog/src/watchdog.py#L416