Enhance Cinder Volume Backup functionality

Roman Hrosrequested to merge
fix/cinder-volume-backup-2 into devel

This MR is based on !3145 (closed), and multiple functionalities/fixes were added:

  • Allow backup to access multiple backends (by default all)
  • Mount additional configs (useful for the backup configuration, such as backup_ceph_conf or backup_gcs_credential_file)
  • Fix getting rbd_cluster_name (used in keyring path)
  • No need for having same names between backups/backends

During backup/restore testing(mainly ceph/s3/gcs) I also found multiple bugs:

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