Support creating users with a default_project_id

Add support in the client to pass the default_project_id attribute.
Also adding a test to make sure that such users automatically get a token
that is scoped to their project - even when requesting an unscoped token.

Change-Id: If4d04f1ac2908409d86e719607f9fa0b1241b997
Merge "Support creating users with a default_project_id"