feat(yamllint): include for this repo and apply rules throughout

* Semi-automated using `ssf-formula` (v0.5.0)
* Fix errors shown below:

```bash
sudoers-formula$ $(grep "\- yamllint" .travis.yml | sed -e "s:^\s\+-\s\(.*\):\1:")
pillar.example
  6:23      warning  truthy value should be one of [false, true]  (truthy)

test/salt/pillar/kitchen.sls
  3:1       warning  missing document start "---"  (document-start)
  7:1       error    too many blank lines (1 > 0)  (empty-lines)

test/salt/pillar/default.sls
  1:1       warning  missing document start "---"  (document-start)
  3:23      warning  truthy value should be one of [false, true]  (truthy)
```

pillar.example

@@ -3,7 +3,7 @@
 ---
 sudoers:
   # By default the main sudoers file is managed by this formula (False to skip)
-  manage_main_config: True
+  manage_main_config: true
   users:
     johndoe:
       - 'ALL=(ALL) ALL'