Make testing neutron agents optional

Closes-Bug: #1247778
Change-Id: Ia67213d661a8dd3b64ca7a7e596e10d3eeba71dc
keystone trusts API test, move delete into tearDown handler

We want to prove the delete works for the various configurations of
trusts created in the tests, so move the delete into tearDown instead
of having a cleanup handler and explicit testing of delete in various
tests, and add a comment explaining why we do it like that.

Change-Id: I32f9aee5657ae54c1ca600f6ebe70bda81cd40e2
Merge "keystone trusts API test, move delete into tearDown handler"
Merge "Make testing neutron agents optional"