rose edit: fix macro metadata setting for section

This fixes setting a `macro=my_module.MacroTransformClass` metadata option on a section.

@arjclark, please review.

This can be tested by setting the contents of `demo/rose-config-edit/demo_meta/app/05-validate/meta/rose-meta.conf` to read:
```
[env]
macro=fib.FibonacciChecker
```

and then launching rose edit for that app.