Switch stable ansible job to 2.8

Ansible 2.6 is EOL. Run tests with 2.8.

Change-Id: I9695af073a855dd0284fc5e8161df79db09cf313

.zuul.yaml

@@ -327,6 +327,27 @@
       test_matrix_branch: master
       tox_install_siblings: true
 
+- job:
+    name: openstacksdk-ansible-stable-2.8-functional-devstack
+    parent: openstacksdk-ansible-functional-devstack
+    description: |
+      Run openstacksdk ansible functional tests against a master devstack
+      using git stable-2.8 branch version of ansible.
+    branches: ^(stable-2.8|master)$
+    required-projects:
+      - name: github.com/ansible/ansible
+        override-checkout: stable-2.8
+      - name: openstack/openstacksdk
+        override-checkout: master
+      - name: openstack/devstack
+        override-checkout: master
+    vars:
+      # test-matrix grabs branch from the zuul branch setting. If the job
+      # is triggered by ansible, that branch will be devel which doesn't
+      # make sense to devstack. Override so that we run the right thing.
+      test_matrix_branch: master
+      tox_install_siblings: true
+
 - job:
     name: openstacksdk-functional-devstack-masakari
     parent: openstacksdk-functional-devstack-minimum
@@ -383,7 +404,7 @@
       jobs:
         - openstacksdk-ansible-devel-functional-devstack:
             voting: false
-        - openstacksdk-ansible-stable-2.6-functional-devstack:
+        - openstacksdk-ansible-stable-2.8-functional-devstack:
             voting: false
         - openstacksdk-functional-devstack
         - openstacksdk-functional-devstack-networking