Validate existence of user_id or email for new user creation

[nurazem]

When creating a new user exactly one of user_id and email has to be given

[dannyfallon]

There are no validation rules in the DSL. The API will return an error when a user ID or email is not provided. To implement client-side checks is doubling up on validation (once client side, once server side). The authoritative validator is the API, I think it best that we let it do its job so I don't see this as a problem.