Standardize CLI commands

[ThomasDelteil]

Feel free to close if you disagree but since MMS is running in the background as daemon, I think it would be clearer if it followed the more common usage:

mxnet-model-server start rather than mxnet-model-server ... mxnet-model-server stop rather than mxnet-model-server --stop

Eventually a: mxnet-model-server restart

[vdantu]

We had a round of discussion and decided to keep "--start/--stop" optional. Its currently not mandatory. We stuck to "--" because its more python'ic way to describe arguments.

[ThomasDelteil]

@vdantu thanks I see, however I think that MMS being what it is now and where it is going, isn't so tied to python anymore. Maybe worth re-considering in the future 👍