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/mitchellh/mapstructure/README.md

@@ -0,0 +1,46 @@
+# mapstructure
+
+mapstructure is a Go library for decoding generic map values to structures
+and vice versa, while providing helpful error handling.
+
+This library is most useful when decoding values from some data stream (JSON,
+Gob, etc.) where you don't _quite_ know the structure of the underlying data
+until you read a part of it. You can therefore read a `map[string]interface{}`
+and use this library to decode it into the proper underlying native Go
+structure.
+
+## Installation
+
+Standard `go get`:
+
+```
+$ go get github.com/mitchellh/mapstructure
+```
+
+## Usage & Example
+
+For usage and examples see the [Godoc](http://godoc.org/github.com/mitchellh/mapstructure).
+
+The `Decode` function has examples associated with it there.
+
+## But Why?!
+
+Go offers fantastic standard libraries for decoding formats such as JSON.
+The standard method is to have a struct pre-created, and populate that struct
+from the bytes of the encoded format. This is great, but the problem is if
+you have configuration or an encoding that changes slightly depending on
+specific fields. For example, consider this JSON:
+
+```json
+{
+  "type": "person",
+  "name": "Mitchell"
+}
+```
+
+Perhaps we can't populate a specific structure without first reading
+the "type" field from the JSON. We could always do two passes over the
+decoding of the JSON (reading the "type" first, and the rest later).
+However, it is much simpler to just decode this into a `map[string]interface{}`
+structure, read the "type" key, then use something like this library
+to decode it into the proper structure.