Lint YAML files under test/

This commit extends YAML linting by enabling standard rules from the
`yamllint` tool [1]. Since syntax errors and key duplicates are already
checked since 4d48711, this change only adds detection for cosmetic
problems. It also narrows checks to the test/ dir only.

The main goal is to prevent future problems to enter the code base
without being noticed. While it would be a huge effort to be PEP8
compliant, it is relatively easy to have correct YAML style *now* and
prevent future errors by enabling linting.

Note: for those (like me) caring about code attribution: use `git blame
-w` to ignore whitespace-only changes.

Note: I disabled some linting checks (such as indentation), they can be
enforced in the future if needed. Similarly, current checks can also be
disabled. See the `.yamllint` file.

[1]: https://yamllint.readthedocs.io/

.yamllint

@@ -1,22 +1,19 @@
 extends: default
 
-# Disable all cosmetic rules
-# (see https://github.com/ansible/ansible/pull/15470#issuecomment-214437876)
-# Only keep 'key-duplicates' and 'new-lines: {type: unix}' checks enabled.
-
 rules:
-  braces: disable
-  brackets: disable
-  colons: disable
-  commas: disable
+  braces: {max-spaces-inside: 1, level: error}
+  brackets: {max-spaces-inside: 1, level: error}
+  colons: {max-spaces-after: -1, level: error}
+  commas: {max-spaces-after: -1, level: error}
   comments: disable
   comments-indentation: disable
   document-start: disable
-  empty-lines: disable
-  hyphens: disable
+  empty-lines: {max: 3, level: error}
+  hyphens: {level: error}
   indentation: disable
   key-duplicates: enable
   line-length: disable
   new-line-at-end-of-file: disable
   new-lines: {type: unix}
   trailing-spaces: disable
+  truthy: disable

test/utils/shippable/code-smell.sh

@@ -15,7 +15,8 @@ if [ "${install_deps}" != "" ]; then
     pip list
 fi
 
-yamllint .
+yamllint ./test
+
 test/sanity/code-smell/replace-urlopen.sh
 test/sanity/code-smell/use-compat-six.sh
 test/sanity/code-smell/boilerplate.sh