Prepare tempest-slow-parallel job and run periodically

tempest-slow-py3 job run all the slow test serially which
takes lot of time and end up job timeout. This preparing
tempest-slow-parallel job which will run slow tests parallelly
in periodic run. Based on the results, later we can make
tempest-slow-py3 job to run tests in parallel.

Also, run tempest-full-parallel in periodic and based on the
result we can run tempest-full-py3 job scenario tests in parallel.

Relavant-Bug: #2004780

Change-Id: I876dacb40daa384cddc8faae3200cd3d39506ddc
diff --git a/zuul.d/integrated-gate.yaml b/zuul.d/integrated-gate.yaml
index f379041..a67f6b4 100644
--- a/zuul.d/integrated-gate.yaml
+++ b/zuul.d/integrated-gate.yaml
@@ -294,6 +294,13 @@
     vars: *tempest_slow_vars
 
 - job:
+    name: tempest-slow-parallel
+    parent: tempest-slow-py3
+    # This job run slow tests in parallel.
+    vars:
+      tox_envlist: slow
+
+- job:
     name: tempest-cinder-v2-api
     parent: devstack-tempest
     # NOTE(gmann): Cinder v2 APIs are available until