Rename folder to csi-snapshotter

This PR renames the folder for cmd from external-snapshotter to
csi-snapshotter to be consistent with other sidecar containers repo.

cmd/csi-snapshotter/main.go

@@ -0,0 +1,7 @@
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("vim-go")
+}