don't create observer enrollments in foreign root accounts

test plan:
 1. have a student enrolled in courses at multiple institutions
    (with no trust relationship between them)
 2. enable parent self-registration at one institution
 3. sign up a parent at that institution
 4. the newly enrolled parent should only receive course invites
    for that institution, and should not see the other
    institution's courses

fixes CNVS-13421

Change-Id: I20baa6fab1989aeb0a1f4eab02b0ca0660333bf6
Reviewed-on: https://gerrit.instructure.com/35937
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>