What this PR does / why we need it:
when the flexvolumeDirVolumePath is truely existent in the host,
```cmd/kubeadm/app/phases/controlplane``` unit tests will be failed.
Fixes#56189
Release note:
```release-note
NONE
```
If certificates for etcd are located in the same directory or
subdirectories of kubernetes pki directory, don't create separate
volumes and mounts in manifests.
Fixeskubernetes/kubeadm#522
Controller manager needs access to Flexvolume plugin when
using attach-detach controller interface.
This PR adds the host mount path for the default directory of flexvolume
plugins
Fixes https://github.com/kubernetes/kubeadm/issues/410
