Added tempest_black_regex to skip tempest tests

* https://review.openstack.org/547106 adds a black_regex flag to
  tempest run. We can use the same in devstack-tempest base job
  to skip/blacklist tests.

Change-Id: I9ed91b01ddb7ec0a3d23305031afbac7c4b16c55
diff --git a/roles/run-tempest/defaults/main.yaml b/roles/run-tempest/defaults/main.yaml
index 85e94f2..c89eb93 100644
--- a/roles/run-tempest/defaults/main.yaml
+++ b/roles/run-tempest/defaults/main.yaml
@@ -1,3 +1,4 @@
 devstack_base_dir: /opt/stack
 tempest_test_regex: ''
 tox_envlist: smoke
+tempest_black_regex: ''