qa/tasks/radosgw_admin.py: coerce key.name and key.acl to str

qa/tasks/radosgw_admin.py: coerce key.name and key.acl to str

if `key.name` is not set, boto fills it with md5, in that case, it comes
from `base64.b64encode()`. so we need to make sure it's str before
passing it to shell.

the same applies to `key.get_xml_acl()`, as its return value comes
directly from something like
```
        response = self.connection.make_request('GET', self.name, key_name,
                                                query_args=query_args,
                                                headers=headers)
        body = response.read()
        # ...
        return body
```

Signed-off-by: Kefu Chai <kchai@redhat.com>