Add "enable_telnet" to gateway

[starkillerOG]

Apperently you can enable the telnet service on xiaomi gateways to get root acces into the operating system of the gateway.
Have not tested it myself (do not (yet) want to mess with the operating system).
But everything is discussed in this HomeAssistant community post: https://community.home-assistant.io/t/xiaomi-mijia-smart-multi-mode-gateway-zndmwg03lm-support/159586/61

It has been tried and succeded for multiple users as it seems in the community post.

[coveralls]

Coverage decreased (-0.1%) to 74.182% when pulling 4813154 on starkillerOG:patch-12 into b0a28f0 on rytilahti:master.

[starkillerOG]

@syssi do you know if the "disable_telnet_service" command also works (just a good guess)?

[syssi]

I've a spare gateway and will give it a try asap.

[rytilahti]

My lumi.gateway.v3 v1.4.1_150 responds with this for this (and the potential) disable command:

miio.exceptions.DeviceError: {'code': -32601, 'message': 'Method not found.'}

[starkillerOG]

My lumi.gateway.v3 v1.4.1_150 responds with this for this (and the potential) disable command:

miio.exceptions.DeviceError: {'code': -32601, 'message': 'Method not found.'}

Well the gateway.v3 apperently does not support that command, it schould work on the lumi.gateway.mgl03

It is a bit frustrating that all these diffrent gateway models support diffrent commands.....

[starkillerOG]

First https://github.com/rytilahti/python-miio/pull/732/files needs to be merged, since the catched model is used in the log message.