[enh] add test.yamllint - lint yaml files

Usage::

    make test.yamllint

    ./manage test.yamllint

test.yamllint is also added to the `test` makefile target.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

.yamllint.yml

@@ -0,0 +1,16 @@
+extends: default
+
+rules:
+
+  indentation:
+    spaces: 2
+
+  # 120 chars should be enough, but don't fail if a line is longer
+  line-length:
+    max: 120
+    level: warning
+    allow-non-breakable-words: true
+
+  # we don't have multiple document per file
+  document-start: disable
+  document-end: disable