Clarify the guideline of negative tests

We have discussed negative tests at OpenStack Summit Austin before
as https://etherpad.openstack.org/p/newton-qa-negative-testing

The conclusion is
* Keep the existing negative tests
  Tempest should not cover all negative test cases as integration tests.
  Such tests should be covered with function tests in each projects.
  Function tests are implemented in Nova, [Ceilometer -> Gabbi], Barbican
  36 negative tests are used in Refstack as required tests and they are valuable

So we need some guideline about negative tests for our future, and this
patch adds it.

Change-Id: I2ac1a4cf91c9357caca2d490e6bc15699b7f3dad
1 file changed