[BUG] Hostpath not configureable via backup secret

[timuthy]

Describe the bug:
Earlier it was possible to configure the hostPath used for the local backup provider via the referenced secret in etcd.spec.backup.store.secretRef, e.g.:

apiVersion: v1
kind: Secret
metadata:
  name: etcd-backup-local
  namespace: default
stringData:
  "hostPath": "/data/etcd/backup"

Unfortunately, this is not possible anymore since the switch to the component concept with #365, it always takes /etc/gardener/local-backupbuckets as the standard path.

Expected behavior:
The hostPath to be configurable.

Environment (please complete the following information):

• Etcd version/commit ID : v0.12.3