Add a shouldDelete method in the provisioner Qualifer interface

[jian-he]

background describe in #1044

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[jian-he]

@wongma7 , mind helping taking a look ? thanks !

[jian-he]

oh, just realized the lib is moved to https://github.com/kubernetes-sigs/sig-storage-lib-external-provisioner

close this and open a PR there..