Adding target to get the latest mmctl release from GitHub (#13770)

* Updating build and release process to include externally built mmctl

* Missed last commit before reset

* rename script to align with purpose

* Moving get release script to scripts dir

scripts/get_latest_release.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# Time: 2020-01-29 14:49:24
+# Author: https://gist.github.com/lukechilds/a83e1d7127b78fef38c2914c4ececc3c
+get_latest_release() {
+  curl --silent "https://api.github.com/repos/$1/releases/latest" | # Get latest release from GitHub api
+    grep '"tag_name":' |                                            # Get tag line
+    sed -E 's/.*"([^"]+)".*/\1/'                                    # Pluck JSON value
+}
+
+# Usage
+# $ get_latest_release "creationix/nvm"
+# v0.31.4
+# Assume we are in a git dir
+get_latest_release $1