Fix Cinder Backup access to kernel modules (iscsi_tcp issue)

It looks like the only missing part was the actual mount of
/lib/modules

Now Cinder Backup volumes differ from Cinder Volume volumes only
by /etc/target which is not relevant (Cinder Backup does not
provide a target).

Change-Id: Iccf4298c4f9306eb0a95b6712815778555ef44fc
Closes-bug: #1863094

ansible/roles/cinder/defaults/main.yml

@@ -94,6 +94,7 @@ cinder_backup_default_volumes:
   - "{{ node_config_directory }}/cinder-backup/:{{ container_config_directory }}/:ro"
   - "/etc/localtime:/etc/localtime:ro"
   - "/dev/:/dev/"
+  - "/lib/modules:/lib/modules:ro"
   - "/run/:/run/:shared"
   - "cinder:/var/lib/cinder"
   - "{% if enable_iscsid | bool %}iscsi_info:/etc/iscsi{% endif %}"

releasenotes/notes/bug-1863094-1564f489a7eecb28.yaml

+---
+fixes:
+  - |
+    Cinder Backup has now access to kernel modules to load e.g. iscsi_tcp
+    module.
+    `LP#1863094 <https://launchpad.net/bugs/1863094>`