fix: retry to get cloud device list until successfully fetching device online state when the network resumes

[SusanPhevos]

Why

In #1519 , after the router restarts, xiaomi_home refreshes the cloud device list when it detects the network to be reconnected to the internet. However, the http post session fails because of "DNS server returned answer with no data" error. User reloads xiaomi_home manually and the http post session succeed. It seems that DNS request is easily to fail soon after the router restarts.
If xiaomi_home does not get the latest device online state from the cloud device list, all the cloud devices will be shown as offline devices whose properties and events would not be subscribed by xiaomi_home. From the cloud service perspective, the brief disconnection of IoT devices caused by a router reboot is not sufficient to classify the devices as offline. The cloud service continues to regard them as online and therefore does not push a “state online” message to xiaomi_home. So the devices are alway unavaible after the router reboot.

Fixed

• Catch the http post error properly.
• After the network resumes, keep retrying to fetch the device list until it succeeds.
• Use constant value to define timer delay.