Migrate backup related pod spec entries from `spec.podSpec` to `spec.backup.podSpec`

[calind]

We should move spec.podSpec.backupAffinity, spec.podSpec.backupNodeSelector, spec.podSpec.backupPriorityClassName and spec.podSpec.backupTolerations under spec.backup.podSpec.

This, with #726 would move all backup-related stuff under the backup key.