Fix the cinder backup action failed when use external ceph

when cinder use external ceph, and edit the cinder-backup.conf follow
the external-ceph-guide.rst, cinder will backup failed because the
backup_ceph_user is not cinder-backup, the patch to fix it.

Change-Id: Ie7213c1280b2b8fc0a4648d09d9ba132427952aa
Closes-Bug: #1740222

doc/source/reference/external-ceph-guide.rst

@@ -124,7 +124,7 @@ Edit /etc/kolla/config/cinder/cinder-backup.conf with the following content:
 
   [DEFAULT]
   backup_ceph_conf=/etc/ceph/ceph.conf
-  backup_ceph_user=cinder
+  backup_ceph_user=cinder-backup
   backup_ceph_chunk_size = 134217728
   backup_ceph_pool=backups
   backup_driver = cinder.backup.drivers.ceph