_stream.py: Opened workspaces store the project that opened them

This is a part of #222

buildstream/_stream.py

@@ -581,15 +581,7 @@ class Stream():
                     todo_elements = "\nDid not try to create workspaces for " + todo_elements
                 raise StreamError("Failed to create workspace directory: {}".format(e) + todo_elements) from e
 
-            workspaces.create_workspace(target._get_full_name(), directory)
-
-            if not no_checkout:
-                with target.timed_activity("Staging sources to {}".format(directory)):
-                    target._open_workspace()
-
-            # Saving the workspace once it is set up means that if the next workspace fails to be created before
-            # the configuration gets saved. The successfully created workspace still gets saved.
-            workspaces.save_config()
+            workspaces.create_workspace(target, directory, not no_checkout)
             self._message(MessageType.INFO, "Created a workspace for element: {}"
                           .format(target._get_full_name()))