config: turn ConfigUnit into singleton

include/py/homekit/config/config.py

@@ -76,6 +76,13 @@ class BaseConfigUnit(ABC):
 class ConfigUnit(BaseConfigUnit):
     NAME = 'dumb'
 
+    _instance = None
+
+    def __new__(cls, *args, **kwargs):
+        if cls._instance is None:
+            cls._instance = super(ConfigUnit, cls).__new__(cls, *args, **kwargs)
+        return cls._instance
+
     def __init__(self, name=None, load=True):
         super().__init__()