trusts API test, avoid creating duplicate user

We currently call create_trustor_and_roles twice, in the base
class and the test class, which is wrong and results in the test
leaving a user behind which is not cleaned up.  So only call it
in the test class, because we need self._interface to be set.

Change-Id: I9b96ae4435714d17bfd2b0b2abe570d98e61bfa6
Closes-Bug: #1267704
1 file changed