add a place for functional test to block specific regressions

We get some bugs reported in by users which we can replicate and fix
most easily in a full stack way. Having a dedicated place to put these
seems like a good idea. They are in no way an attempt to test all the
things, they are specific tests that demonstrate a bug, and ensure
that once that particular bug is fixed, it will not come back.

Change-Id: Idfd8680133197beabaf6e1ac8df7490c2a345b17
Related-Bug: #1522536

nova/tests/functional/regressions/README.rst

@@ -0,0 +1,24 @@
+================================
+ Tests for Specific Regressions
+================================
+
+When we have a bug reported by end users that we can write a full
+stack reproduce on, we should. And we should keep a regression test
+for that bug in our tree. It can be deleted at some future date if
+needed, but largely should not be changed.
+
+Writing Regression Tests
+========================
+
+- These should be full stack tests which inherit from
+  nova.test.TestCase directly. (This is to prevent coupling with other tests).
+
+- They should setup a full stack cloud in their setUp via fixtures
+
+- They should each live in a file which is named test_bug_######.py
+
+Writing Tests Before the Bug is Fixed
+=====================================
+
+TODO describe writing and landing tests before the bug is fixed as a
+reproduce.