Issue title: Move from glide to dep
 Issue body: dep is production-ready and supposedly better. it also has vendor pruning built in so we can use that instead of having people use the third-party glide-vc tool to keep the vendor dir size down
 Comments: 
 Comment 0: I'm glad to help
 Comment 1: fyi @MartinForReal https://github.com/kubernetes-incubator/external-storage/pull/533/files was WIP but nobody was around to review it. ping @hzxuzhonghu sorry nobody could review it, we can continue working on it now as I will be available.
 Comment 2: thank you again to @MartinForReal for your offer and @hzxuzhonghu for writing and updating the gopkg.toml, this is done!
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