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