Add support for SSE-S3 server side encryption with vault (#6192)

Add support for sse-s3 encryption with vault as KMS.

Also refactoring code to make use of headers and functions defined in
crypto package and clean up duplicated code.

vendor/github.com/hashicorp/hcl/testhelper/unix2dos.go

@@ -0,0 +1,15 @@
+package testhelper
+
+import (
+	"runtime"
+	"strings"
+)
+
+// Converts the line endings when on Windows
+func Unix2dos(unix string) string {
+	if runtime.GOOS != "windows" {
+		return unix
+	}
+
+	return strings.Replace(unix, "\n", "\r\n", -1)
+}