tests: CentOS 6 lacks %wheel in sudo by default.

tests/image_prep/_container_setup.yml

@@ -2,7 +2,6 @@
 - hosts: all
   strategy: linear
   gather_facts: false
-  become: true
   tasks:
     - raw: >
         if ! python -c ''; then
@@ -96,6 +95,11 @@
         dest: /etc/sudoers.d/001-mitogen
         src: ../data/docker/001-mitogen.sudo
 
+    - lineinfile:
+        path: /etc/sudoers
+        line: "%wheel ALL=(ALL) ALL"
+      when: distro == "CentOS"
+
     - lineinfile:
         path: /etc/ssh/sshd_config
         line: Banner /etc/ssh/banner.txt