Add single string shorthand for Top File

As per issue #5804, this makes the following valid syntax for the `top.sls` file:

```yaml
base:
  tommy.example.com:   webserver
  jerry.example.com:   dbserver
  alberon.example.com: mailserver

develop:
  saturn.example.com:  webserver
```

I added a test and documentation to the Top File reference.