Use default global-backup reference for k8up s3 access and secret key

[TheBigLee]

Context

Currently the default vault reference for the s3 access and secret keys are

accesskey: '?{vaultkv:${customer:name}/${cluster:name}/vault/${_instance}/backup/s3_access_key}'
secretkey: '?{vaultkv:${customer:name}/${cluster:name}/vault/${_instance}/backup/s3_secret_key}'

This component relies on having the k8up component installed. The s3 access and secret key are already stored in vault for k8up itself to work. Instead of duplicating those keys in vault, we should leverage the existing keys and default to those keys:

backup:                                                                              
  bucket:                                                                            
    accesskey: '?{vaultkv:${customer:name}/${cluster:name}/global-backup/access-key}'
    secretkey: '?{vaultkv:${customer:name}/${cluster:name}/global-backup/secret-key}'