Check that ansible_distribution_major_version is defined in sudo task

This variable does not exist when run on OpenBSD servers, making the ansible
playbook to exit in a fatal state.

evolinux-users/tasks/sudo.yml

@@ -4,6 +4,6 @@
   when: ansible_distribution_release == "jessie"
 
 - include: sudo_stretch.yml
-  when: ansible_distribution_major_version is version('9', '>=')
+  when: ansible_distribution_major_version is defined and ansible_distribution_major_version is version('9', '>=')
 
 - meta: flush_handlers