Support new or empty role for XSEDE / RMACC users

[anderbubble]

XSEDE doesn't inform us of a requesting user's role, so if we want to track it, we'll have to do it by hand as a post-process (or give the user a way to populate it after account creation).

Until then, we need a placeholder role to use for new XSEDE accounts. It's defaulting to "student" right now.

Potential suggestions:

• rmacc
• xsede
• unknown
• None / null

[anderbubble]

I talked about this a little with Joel, but any additional feedback would be welcomed here as well.