Remove roles from OS-TRUST list responses

According to the docs, the list responses should not contain
the roles, only the detailed response when you get a trust
explicitly by ID.  So remove the roles and modify the tests
appropriately.

Note it was also observed that expires_at is present in all
GET resonses, but not in the docs, but this has been agreed
as a docs error so will be addressed via a docs patch.

Change-Id: I5387021a53f3284add9e5e71e9e005c4dd31b76c
Closes-Bug: #1245590
Cleanup duplication in test_backend

There was some code duplicated in test_backend so this removes
the duplication by providing a function.

Change-Id: I6635f98932812fe2adae04feed9c07f3aa81eb5d
Merge "Cleanup duplication in test_backend"
Merge "Remove roles from OS-TRUST list responses"