Merge the relation data from all kube-control endpoints rather than c…

…ounting only on the first endpoint

ops/ops/interface_kube_control/requires.py

@@ -41,8 +41,10 @@ def relation(self) -> Optional[Relation]:
     @cached_property
     def _data(self) -> Optional[Data]:
         if self.relation and self.relation.units:
-            raw_data = self.relation.data[list(self.relation.units)[0]]
-            return Data(**raw_data)
+            rx = {}
+            for unit in self.relation.units:
+                rx.update(self.relation.data[unit])
+            return Data(**rx)
         return None
 
     def evaluate_relation(self, event) -> Optional[str]: