Allow listing a single module in a top match

This makes the following valid syntax for the top.sls file:

base:
  tommy.example.com:   webserver
  jerry.example.com:   dbserver
  alberon.example.com: mailserver

develop:
  saturn.example.com:  webserver
Add documentation for shorthand syntax
Merge pull request #5929 from jcassee/feature/top_scalars

Add single string shorthand for Top File