Make heat-slow job run in parallel

The heat slow job was running it's tests serially. This patch changes
that so the tests will run in parallel to enable faster execution.

Change-Id: I55fe85952c13b2b40b2b9f5b4dd6073b21b7a3ea
diff --git a/tox.ini b/tox.ini
index 4a625f8..5e8d283 100644
--- a/tox.ini
+++ b/tox.ini
@@ -54,7 +54,7 @@
 setenv = OS_TEST_TIMEOUT=1200
 # The regex below is used to select heat api/scenario tests tagged as slow.
 commands =
-  bash tools/pretty_tox_serial.sh '(?=.*\[.*\bslow\b.*\])(^tempest\.(api|scenario)\.orchestration) {posargs}'
+  bash tools/pretty_tox.sh '(?=.*\[.*\bslow\b.*\])(^tempest\.(api|scenario)\.orchestration) {posargs}'
 
 [testenv:large-ops]
 sitepackages = True