openstacksdk's Credential expects user_id and project_id, not user and
project. Previously, we would send payloads like
{'type': 'ec2', 'blob': '{"access": "s3-user1", "secret": "s3-secret1"}'}
which Keystone would reject with
'user_id' is a required property
Change-Id: I0544bef7df9247395f0726ea075112d6ac992252
1979c20ff0