openstack_config: fix docker exec command

container_exec_cmd should be replace by docker_exec_cmd.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1765110

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>

roles/ceph-osd/tasks/openstack_config.yml

 ---
 - name: wait for all osd to be up
-  command: "{{ container_exec_cmd | default('') }} ceph --cluster {{ cluster }} -s -f json"
+  command: "{{ hostvars[groups[mon_group_name][0]]['docker_exec_cmd'] | default('') }} ceph --cluster {{ cluster }} -s -f json"
   register: wait_for_all_osds_up
   retries: "{{ nb_retry_wait_osd_up }}"
   delay: "{{ delay_wait_osd_up }}"