trustee unable to perform role based operations on trust

There was a typo in a conditional that checked the trustor twice, instead
of trustee and trustor. The typo was preventing a trustee from being
able to list or get or check roles that were delegated.

Also, removed the spurious check of is_admin from said conditional
(_is_trustor_trustee_admin), and renamed it to _is_trustor_trustee.

An additional check, to see if a trust existed, was removed since it
was repeated unnecessarily.

Added a test to cover the gap that was discovered in the bug report.

SecurityImpact

Change-Id: Id61e71cb3ea7c6bc51783af067fbbbe4c9bffafe
Closes-Bug: 1261104
Merge "trustee unable to perform role based operations on trust"