set old backupConfig secretKey if it doesn't exist in data
set old backupConfig secretKey if it doesn't exist in data

Issue: UI currently updates cluster with replace=true, which means
if secretKey is not passed in backupConfig, it gets reset.

cjellick/rancher,rancher/rancher,rancher/rancher,rancher/rancher,rancherio/rancher,cjellick/rancher,cjellick/rancher,rancherio/rancher,rancher/rancher