Fix 'nova_cephx_raw_key' is undefined

When using external ceph without nova integration, kolla-ansible fails
because 'nova_cephx_raw_key' is undefined.
This patch fixes the issue by applying the 'default' filter to
prevent failure on undefined.
The change in behavior was introduced by [1].

[1] https://review.opendev.org/689753
commit 44709f413246a05b891f250e88c43226e1f1b0c9

Change-Id: I2fdca1a6a78e78623733a387a2d8c7e29d449083
Closes-Bug: #1853862
Co-Authored-By: Mark Goddard <mark@stackhpc.com>

ansible/roles/nova-cell/tasks/external_ceph.yml

@@ -111,11 +111,13 @@
     - item.enabled | bool
     - external_ceph_cephx_enabled | bool
   with_items:
+    # NOTE(yoctozepto): 'default' filter required due to eager evaluation of item content
+    # which will be undefined if the applicable condition is False
     - uuid: "{{ rbd_secret_uuid }}"
-      result: "{{ nova_cephx_raw_key }}"
+      result: "{{ nova_cephx_raw_key | default }}"
       enabled: "{{ nova_backend == 'rbd' }}"
     - uuid: "{{ cinder_rbd_secret_uuid }}"
-      result: "{{ cinder_cephx_raw_key }}"
+      result: "{{ cinder_cephx_raw_key | default }}"
       enabled: "{{ cinder_backend_ceph }}"
   notify:
     - Restart nova-libvirt container