run-tempest role: new tempest_test_regex variable

Specify a regular expression to filter the tests.

Change-Id: I44911c6bd6ede68c3376c8a6b42700310dce7a06
diff --git a/roles/run-tempest/defaults/main.yaml b/roles/run-tempest/defaults/main.yaml
index e1e81da..3e57511 100644
--- a/roles/run-tempest/defaults/main.yaml
+++ b/roles/run-tempest/defaults/main.yaml
@@ -1,2 +1,3 @@
 devstack_base_dir: /opt/stack
+tempest_test_regex: ''
 tox_venvlist: smoke