elevates privileges and grants access to /dev/mapper to cinder backup

- sets privileged: true on cinder backup container
- adds /dev/mapper to cinder-backup mounts
- this change allows the cinder backup manager to access the
  cinder-volumes lvm volume group via the kernel device mapper.

Change-Id: I252ee9cadb5f06c383ac9f48d0b1b16d83037a41
Closes-Bug: #1542574

ansible/roles/cinder/tasks/start.yml

@@ -16,9 +16,11 @@
     common_options: "{{ docker_common_options }}"
     name: "cinder_backup"
     image: "{{ cinder_backup_image_full }}"
+    privileged: True
     volumes:
       - "{{ node_config_directory }}/cinder-backup/:{{ container_config_directory }}/:ro"
       - "/run/kolla/log:/dev/log"
+      - "/dev/mapper/:/dev/mapper/"
   when: inventory_hostname in groups['cinder-backup']
 
 - name: Starting cinder-scheduler container