Automatic merge from submit-queue (batch tested with PRs 58480, 58549). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Use backup location to load cloud config for OpenStack

**What this PR does / why we need it**:

Since we are transitioning to external cloud provider, we need a way
to use the existing cinder volume plugin (from kubelet). With external
cloud manager kubelet will be run with --cloud=provider=external and
no --cloud-config file will be in the command line. So we need a way
to load the openstack config file from somewhere.

Taking a cue from kubeadm, which currently is picking up "/etc/kubernetes/cloud-config"
https://github.com/kubernetes/kubernetes/blob/master/cmd/kubeadm/app/phases/controlplane/manifests.go#L44

let's support the scenario where we fall back to this static location if
there is no cloud provider specified in the command line.

This has been tested with local-up-cluster using the following params:
EXTERNAL_CLOUD_PROVIDER=true
CLOUD_PROVIDER=openstack
CLOUD_CONFIG=/etc/kubernetes/cloud-config


**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```