Enforce scope mutual exclusion for trusts

We already warn if you try to scope a domain and project together. This
should be extended to trust scoping rather than clobbering the existing
scope.

Change-Id: I9d8fe001b65588b1c21e58f38a47456fdad85ee1
Related-Bug: #1288223
Merge "Enforce scope mutual exclusion for trusts"
Test commit