Fix stacktrace when exiting with Ctrl + C

Fix #547

gns3server/controller/__init__.py

@@ -125,10 +125,17 @@ class Controller:
         :param compute_id: Compute server identifier
         """
         compute = self.get_compute(compute_id)
+        yield from compute.close()
         del self._computes[compute_id]
         self.save()
         self.notification.emit("compute.deleted", compute.__json__())
 
+    @asyncio.coroutine
+    def close(self):
+        log.info("Close controller")
+        for compute in self._computes.values():
+            yield from compute.close()
+
     @property
     def notification(self):
         """

gns3server/controller/compute.py

@@ -93,6 +93,11 @@ class Compute:
         self._connected = False
         self._controller.notification.emit("compute.updated", self.__json__())
 
+    @asyncio.coroutine
+    def close(self):
+        self._connected = False
+        self._session.close()
+
     @property
     def version(self):
         """