Atomic save of the .gns3

Fix #579

gns3server/controller/project.py

@@ -363,8 +363,9 @@ class Project:
             topo = project_to_topology(self)
             path = self._topology_file()
             log.debug("Write %s", path)
-            with open(path, "w+") as f:
+            with open(path + ".tmp", "w+") as f:
                 json.dump(topo, f, indent=4, sort_keys=True)
+            shutil.move(path + ".tmp", path)
         except OSError as e:
             raise aiohttp.web.HTTPInternalServerError(text="Could not write topology: {}".format(e))