Draft: Resolve "Fail if devices request mixed green modes in single device server"

Closes #452 (closed)

The new _get_green_mode_from_classes_or_global_setting method was added, that checks wheter and which green_mode was used for each device in server.

After that the logic is the following:

If there is only one device and it specifies its green mode explicitly, then the global green mode is ignored.
When in one server there are devices with different modes.

The corresponding tests were added.