Use '.yaml' extension as default, not '.yml'

As someone said [1] on the internet:

    Say ".yaml" not ".yml".
    This is not MS-DOS, and YML is a Yahoo XML dialect.

Similarly, we use '.json', not '.jsn'.

[1]: https://github.com/ceph/s3-tests/commit/e17c56a

yamllint/conf/relaxed.yaml

@@ -0,0 +1,27 @@
+---
+
+extends: default
+
+rules:
+  braces:
+    level: warning
+    max-spaces-inside: 1
+  brackets:
+    level: warning
+    max-spaces-inside: 1
+  colons:
+    level: warning
+  commas:
+    level: warning
+  comments: disable
+  comments-indentation: disable
+  document-start: disable
+  empty-lines:
+    level: warning
+  hyphens:
+    level: warning
+  indentation:
+    level: warning
+    indent-sequences: consistent
+  line-length:
+    level: warning