Use gsutil rsync instead of cp for buckets

[nader-ziada]

Changes

Fixes #646

Use gsutil rsync instead of cp for buckets to handle the case when the bucket is empty

cp has an a bug where it will fail, but rsync works fine

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Release Notes

Use gsutil rsync instead of cp for updating storage resource, handling the case where the bucket is empy and `dir` type is used.

[vdemeester]

/lgtm

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