Add documentation about white box/black box testing to HACKING

This patch set adds documentation about white box vs block box
testing and their relationship in Patrole. This is so that
devs/test writers understand that Patrole is a bit different than
Tempest and requires digging a bit deeper in the internals of
the API implementation in order to properly test RBAC.

Also removes a misleading link in the README.rst section. The
discussion on member vs. _member_ role is very outdated and
so a link is provided to the RBAC overview section instead which
is concerned with documenting such information.

Change-Id: I0a014c2e917caeb058dd5b5294dd0af2e5e49132
3 files changed