refactored lti_role_creator into lti_user_creator

Change-Id: Ide94a66bd850a2c6e60ae9353192257c97e2a5db
Reviewed-on: https://gerrit.instructure.com/29977
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stephan Hagemann <stephan@pivotallabs.com>
Product-Review: Stephan Hagemann <stephan@pivotallabs.com>
QA-Review: Stephan Hagemann <stephan@pivotallabs.com>