engine: resources, graph: Change the Close method to Cleanup

This also changes a few similarly named methods. Clearer what it's doing
in terms of cleanup vs. causing some action.

engine/resources/group.go

@@ -71,8 +71,8 @@ func (obj *GroupRes) Init(init *engine.Init) error {
 	return nil
 }
 
-// Close is run by the engine to clean up after the resource is done.
-func (obj *GroupRes) Close() error {
+// Cleanup is run by the engine to clean up after the resource is done.
+func (obj *GroupRes) Cleanup() error {
 	return nil
 }