zuul: fix the parent of the packstack jobs, concurrency

Needed after https://review.openstack.org/517644 which changed the
definition of the base jobs (especially where the type of the base
image is defined).
Reduce the concurrency of tempest run to (hopefully) prevent some
errors related to networking.

Change-Id: Ifc0d186bc23d318fab994b99cb94f1e587d41d8f

.zuul.yaml

@@ -76,7 +76,7 @@
 
 - job:
     name: python-tempestconf-tempest-packstack-admin
-    parent: packstack-base
+    parent: packstack-allinone
     description: |
       Tempest job for python-tempestconf on a packstack environment as the admin user.
     required-projects:
@@ -101,7 +101,7 @@
 
 - job:
     name: python-tempestconf-tempest-packstack-demo
-    parent: packstack-base
+    parent: packstack-allinone
     description: |
       Tempest job for python-tempestconf on a packstack environment as the demo user.
     required-projects:

playbooks/python-tempestconf-tempest-packstack.yaml

@@ -39,3 +39,5 @@
     - name: Run Tempest Tests
       include_role:
         name: run-tempest
+      vars:
+        tempest_concurrency: 2