tests/rgw: use json format output for user info

tests/rgw: use json format output for user info

If the radosgw user already exists then we need to have the output in json
format because we are expecting to load the output with json.loads()
Otherwise we have pytest failure like:

```console
self = <json.decoder.JSONDecoder object at 0x7fa2f00a5fd0>, s = '', idx = 0

    def raw_decode(self, s, idx=0):
        """Decode a JSON document from ``s`` (a ``str`` beginning with
        a JSON document) and return a 2-tuple of the Python
        representation and the index in ``s`` where the document ended.

        This can be used to decode a JSON document from a string that may
        have extraneous data at the end.

        """
        try:
            obj, end = self.scan_once(s, idx)
        except StopIteration as err:
>           raise JSONDecodeError("Expecting value", s, err.value) from None
E           json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>