[doc] Ensure that the reason for using the role link mechanism in role tests is documented

[Jesse Pretorius]

In various changes to the role tests we've implemented a mechanism as represented in https://review.openstack.org/370246 which links the toxinidir into the ansible roles directory being used for the tests.

This is being done for the following reasons:

[17:01:08] automagically: cloudnull: The role link changes are because of the ordering of items within the ANSIBLE_ROLE_PATH var correct?
[17:01:26] automagically: i.e. there is some risk that the role could be found in the first path
[17:01:31] @cloudnull: yes.
[17:01:44] automagically: Nice catch, subtle problem that would cause some real issues if triggered
[17:01:50] @cloudnull: also if there's an existing clone of the role on the disk it'll use that instead
[17:02:10] @cloudnull: so yea, it's a minimal risk

[17:50:14] odyssey4me: automagically cloudnull the role linking will actually make role test execution a bit easier
[17:50:32] odyssey4me: we can just target the role name as it normally is, instead of toxinidir
[17:50:39] automagically: odyssey4me: Yeah, we should be able to remove the -e role_name business now
[17:50:56] odyssey4me: we should note that somewhere actually
[17:51:04] odyssey4me: somewhere in the test role
[17:51:23] odyssey4me: ie note that the execution of the role with the role name relies on the role link mechanism being in place via tox

The reason should be noted in the tests repository somewhere so that this decision is recorded with its reasons, seeing as the commits did not cover the reasons.

[Jesse Pretorius (jesse-pretorius)]

This has been done in the central Ansible environment preparation script: https://review.openstack.org/375061