Add telnet_password for ShellE1 and ShellMGW2

[xiaodong-lx]

Add telnet_password in ConfigFlow, only tested on Mijia_Hub_V2.
It's not safe to set a empty password for root user, Gateway exposes telnet port in the public network, especially in IPv6 network.

README.md

Telnet Password - gateway(Aqara_Hub_E1 or Mijia_Hub_V2) telnet password. Password will setup while adding integration(keep field empty if you don't want setup password), or you need change both integration config and password in telnet(passwd or chpasswd) later.

Config

Issues

• Set telnet password on Gateway 3 #625
• Request to add support for the Telnet password of the gateway. #1304

Known issue

If telnet_password is configured and not empty, user can not reconfigure telnet_password to empty, I'm not familiar with Python, maybe you can fix it. :-)