Add external-storage tests for nfs

[Madhu-1]

We have external-storage tests for cephfs https://github.com/ceph/ceph-csi/tree/devel/scripts/k8s-storage not for nfs, This is a tracker for nfs tests

Tasks

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1. add driver-nfs.yaml in scripts/k8s-storage directory
2. add sc-nfs.yaml.in in scripts/k8s-storage directory
3. verify the output of a manual trigger CI run that NFS tests are executed