Make snippets follow standards

Update the snippets to follow the best practices mentioned in the [styleguide](http://docs.puppetlabs.com/guides/style_guide.html). Updates include:

- Ensuring all strings are single quoted by default.
- All inner items end the line with a comma.
- Default file mode is a 4 digits, not three.
- Various fixes to value substitution/completions to prevent throwing empty
  strings.