Enable btrfs/fuse-overlayfs/stargz snapshotter plugins

Signed-off-by: Brad Davidson <brad.davidson@rancher.com>

vendor/github.com/hashicorp/go-cleanhttp/README.md

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+# cleanhttp
+
+Functions for accessing "clean" Go http.Client values
+
+-------------
+
+The Go standard library contains a default `http.Client` called
+`http.DefaultClient`. It is a common idiom in Go code to start with
+`http.DefaultClient` and tweak it as necessary, and in fact, this is
+encouraged; from the `http` package documentation:
+
+> The Client's Transport typically has internal state (cached TCP connections),
+so Clients should be reused instead of created as needed. Clients are safe for
+concurrent use by multiple goroutines.
+
+Unfortunately, this is a shared value, and it is not uncommon for libraries to
+assume that they are free to modify it at will. With enough dependencies, it
+can be very easy to encounter strange problems and race conditions due to
+manipulation of this shared value across libraries and goroutines (clients are
+safe for concurrent use, but writing values to the client struct itself is not
+protected).
+
+Making things worse is the fact that a bare `http.Client` will use a default
+`http.Transport` called `http.DefaultTransport`, which is another global value
+that behaves the same way. So it is not simply enough to replace
+`http.DefaultClient` with `&http.Client{}`.
+
+This repository provides some simple functions to get a "clean" `http.Client`
+-- one that uses the same default values as the Go standard library, but
+returns a client that does not share any state with other clients.