upstream CI: Use fedora-latest as default test container.

With the removal of CentOS 8 container, the available Fedora latest
image will be used for tests, if a specific container is not given.

tests/azure/templates/group_tests.yml

@@ -2,7 +2,7 @@
 parameters:
   - name: scenario
     type: string
-    default: centos-8
+    default: fedora-latest
   - name: build_number
     type: string
   - name: ansible_version

tests/azure/templates/playbook_tests.yml

@@ -8,7 +8,7 @@ parameters:
     default: 1
   - name: scenario
     type: string
-    default: centos-8
+    default: fedora-latest
   - name: ansible_version
     type: string
     default: ""

tests/azure/templates/pytest_tests.yml

@@ -4,6 +4,7 @@ parameters:
     type: string
   - name: scenario
     type: string
+    default: fedora-latest
   - name: ansible_version
     type: string
     default: ""