For now, effectively there could be multiple trusts with the same
project, trustor, trustee, expiry date, impersonation. The same
combination can have multiple trusts assigned with different roles
or not.
Patch fixes this issue by adding unique constraint to the trusts
database model. If two requests create trusts with the same
trustor, trustee, project, expiry, impersonation, then the second
request would bring up an exception saying there's a conflict.
This can help to improve specific trusts identification and
improve user experience.
Change-Id: I1a681b13cfbef40bf6c21271fb80966517fb1ec5
Closes-Bug: #1475091
59b09b50ff