Add py3 prefix to Python 3 platform vars files

Change-Id: I577f2558bb1468478fa6c86b65646286a49fffac

roles/tobiko-ensure-python3/tasks/platform.yaml

@@ -13,11 +13,11 @@
   include_vars: "{{ item }}"
   ignore_errors: yes
   with_first_found:
-    - "{{ ansible_distribution }}-{{ ansible_distribution_major_version }}.yaml"
-    - "{{ ansible_os_family }}-{{ ansible_distribution_major_version }}.yaml"
-    - "{{ ansible_distribution }}.{{ ansible_architecture }}.yaml"
-    - "{{ ansible_distribution }}.yaml"
-    - "{{ ansible_os_family }}.yaml"
+    - "py3-{{ ansible_distribution }}-{{ ansible_distribution_major_version }}.yaml"
+    - "py3-{{ ansible_os_family }}-{{ ansible_distribution_major_version }}.yaml"
+    - "py3-{{ ansible_distribution }}.{{ ansible_architecture }}.yaml"
+    - "py3-{{ ansible_distribution }}.yaml"
+    - "py3-{{ ansible_os_family }}.yaml"
   register: include_platform_vars