s3fs: Add defaults file incl. example

s3fs/defaults/main.yml

+---
+
+s3fs: []
+
+# s3fs:
+#   - bucket: example
+#     # See s3fs(1) for further options.
+#     opts:
+#       - url=http://ceph-rgw.example.com:7480
+#       - use_path_request_style
+#       - _netdev
+#       accesskey: 26W1AVB409SC3T0VJ10T
+#       secret: hunter2
+#     # mountpoint: optional, defaults to bucket name