Fix Options Flow for Gateway and Sub Devices

[alexualbu]

Draft PR to fix Options flow for Gateways and Sub-devices. First contribution so please bear with me & have a first look @leeyuentuen . Should I add and issue first?

Issues fixed:

• options flow does not work for gateways due to missing entities (#28)
• init step of options flow for sub-devices required both host and local key + not saving parent_gateway which caused issues with the devices upon saving

[leeyuentuen]

on first view it look ok.
if you want i can merge them and put them in beta to let other people test them

[alexualbu]

thanks, @leeyuentuen!
So are you saying I should create another beta branch ?

[leeyuentuen]

i've created a beta release on it.
https://github.com/leeyuentuen/localtuya/releases/tag/3.6.4-beta.1