Refactor test_auth trust related tests

The tests in the AuthWithTrust fixture had many dependencies on global
variables that were not necessarily used; for instance, every time the
setUp function is called, there was a new trust created. This was ran
even for classes that don't need the creation of this initial valid
trust.

This commit gets rid of this behaviour, thus making the test less
dependent on the fixture's state (class variables) in favor of a more
on-demand approach to getting trusts and tokens.

Change-Id: I64752702cc8a328463df15591e3f4c082266ef6d
Add note for v3 API clients using auth plugin docs

Change-Id: I3d07c3f27ac38f9ead0481271460b3b3633b8c1d
Closes-Bug: #1311324
Merge "Refactor test_auth trust related tests"
Merge "Add note for v3 API clients using auth plugin docs"