add lti user, lti roles, lit tool, lit assignment, and lti context creators

also refactored add_variable_mapping to be more explicit

Change-Id: I38b21975ae80cf510f81bfba84ece7b3b568bfa9
Reviewed-on: https://gerrit.instructure.com/29241
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stephan Hagemann <stephan@pivotallabs.com>
Product-Review: Stephan Hagemann <stephan@pivotallabs.com>
QA-Review: Brad Humphrey <brad@instructure.com>