Use project when logging the user identity

The logging_user_identity_format option's default value is changed
to reference the project instead of the tenant. This is necessary
because I2fded6f3476df1fb8c4e042ef28ed8ccb7ab0737 removed the tenant
argument from request contexts.

Related-Bug: #1505827
Change-Id: Id6d4b6eaa2b96ba5bd53e8b2dae5d9eea13b25d9