plugin/check_url_available: rewrite options

The check_url_available promise has been improved with the following
changes:

- The `http_code` parameter has been renamed `http-code` for consistency.
- The `check-secure` parameter has been removed. Users should specify
  `http-code = 401` instead.
- The `username` and `password` parameters have been added to optionally
  enable HTTP basic authentication.
- The logging output has been changed slightly.