Default to Ansible 2.9

Ansible 2.9 is there since several months now and had numerous bugfix
releases. Thus we should default to it since it is the newest and best
maintained release.

Change-Id: If6687514a6237bb9816b54745aac5ec24383c8bf

releasenotes/notes/ansible-29-default-067e82c48805c5d5.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - |
+    Zuul now defaults to Ansible 2.9 if no version is specified.

tests/fixtures/config/ansible-versions/git/common-config/zuul.yaml

@@ -25,7 +25,7 @@
     parent: ansible-version
     vars:
       test_ansible_version_major: 2
-      test_ansible_version_minor: 8
+      test_ansible_version_minor: 9
 
 # This job is used by a test case specifying a different ansible version in
 # zuul.conf

zuul/lib/ansible-config.conf

@@ -1,6 +1,6 @@
 # This file describes the currently supported ansible versions
 [common]
-default_version = 2.8
+default_version = 2.9
 requirements = ara>=0.16.5,<1.0.0 openstacksdk openshift jmespath google-cloud-storage
 
 [2.6]