Switch default ansible version to 2.7

Since 2.5 is unsupported soon, switch the default to 2.7.

Change-Id: I07c3dd7ad39f95d1ba1c18020a1841fcfe2935ab

releasenotes/notes/default-ansible-9983f6e8f31bf9cb.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    The default ansible version is now 2.7. In case this breaks any jobs this
+    can be overridden in :attr:`job.ansible-version`,
+    :attr:`tenant.default-ansible-version` or
+    :attr:`scheduler.default_ansible_version`

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: 5
+      test_ansible_version_minor: 7
 
 - job:
     name: ansible-25

zuul/lib/ansible-config.conf

@@ -1,7 +1,6 @@
 # This file describes the currently supported ansible versions
 
 [2.5]
-default = true
 deprecated = true
 requirements = ansible>=2.5.1,<2.6 ara
 
@@ -9,4 +8,5 @@ requirements = ansible>=2.5.1,<2.6 ara
 requirements = ansible>=2.6,<2.7 ara
 
 [2.7]
+default = true
 requirements = ansible>=2.7,<2.8 ara