Add role parameter to LTI. BLD-583.

Add role parameter to LTI regarding IMS Global Learning Tools Interoperability™ Implementation Guide.

<b>Issue:</b> https://edx-wiki.atlassian.net/browse/BLD-583
<b>Spec</b>: Basic Launch Data http://www.imsglobal.org/LTI/v1p1p1/ltiIMGv1p1p1.html#_Toc330273026 

@auraz and @nedbat please review.