Define ostree variable for runc (#9321)

The ostree variable is not defined previously raising an error when
the runtime tries to read it.

roles/container-engine/runc/tasks/main.yml

 ---
+- name: runc | check if fedora coreos
+  stat:
+    path: /run/ostree-booted
+    get_attributes: no
+    get_checksum: no
+    get_mime: no
+  register: ostree
+
 - name: runc | set is_ostree
   set_fact:
     is_ostree: "{{ ostree.stat.exists }}"