Fix a rare error when closing a project

Fix #897

gns3server/controller/project.py

@@ -536,7 +536,7 @@ class Project:
     @asyncio.coroutine
     def close(self, ignore_notification=False):
         yield from self.stop_all()
-        for compute in self._project_created_on_compute:
+        for compute in list(self._project_created_on_compute):
             try:
                 yield from compute.post("/projects/{}/close".format(self._id), dont_connect=True)
             # We don't care if a compute is down at this step