Zuul: add file extension to playbook path

Zuul now supports including the file extension on the playbook path
and omitting the extension is now deprecrated.  Update references
to include the extension.

Change-Id: Ida928be6977d433216751dbba94c6e3837daf6bc

.zuul.yaml

@@ -3,9 +3,9 @@
     parent: devstack
     description: |
       Base job for devstack-based functional tests
-    pre-run: playbooks/devstack/pre
-    run: playbooks/devstack/run
-    post-run: playbooks/devstack/post
+    pre-run: playbooks/devstack/pre.yaml
+    run: playbooks/devstack/run.yaml
+    post-run: playbooks/devstack/post.yaml
     roles:
       - zuul: openstack-infra/devstack
     timeout: 9000
@@ -25,18 +25,18 @@
       - openstack/tacker-horizon
     vars:
       devstack_services:
-        horizon: True
-        tacker: True
-        tacker-conductor: True
+        horizon: true
+        tacker: true
+        tacker-conductor: true
       devstack_plugins:
         heat: https://git.openstack.org/openstack/heat
-        networking-sfc:  https://git.openstack.org/openstack/networking-sfc
+        networking-sfc: https://git.openstack.org/openstack/networking-sfc
         aodh: https://git.openstack.org/openstack/aodh
         ceilometer: https://git.openstack.org/openstack/ceilometer
         barbican: https://git.openstack.org/openstack/barbican
         mistral: https://git.openstack.org/openstack/mistral
         tacker: https://git.openstack.org/openstack/tacker
-      tox_install_siblings: False
+      tox_install_siblings: false
       tox_envlist: dsvm-functional
       zuul_work_dir: src/git.openstack.org/openstack/tacker