Add "env" template func to get env var value

ssm/transformations/template_funcs.go

@@ -1,12 +1,15 @@
 package transformations
 
 import (
+	"fmt"
 	"net/url"
+	"os"
 	"strings"
 	"text/template"
 )
 
 var funcMap = template.FuncMap{
+	"env":          GetEnv,
 	"url_host":     URLHost,
 	"url_password": URLPassword,
 	"url_path":     URLPath,
@@ -16,6 +19,15 @@ var funcMap = template.FuncMap{
 	"replace":      strings.Replace,
 }
 
+// GetEnv gets the environment variable
+func GetEnv(input string) (string, error) {
+	val, ok := os.LookupEnv(input)
+	if !ok {
+		return "", fmt.Errorf("can't find %s in the environment variables", input)
+	}
+	return val, nil
+}
+
 // URLUser extracts user from the URL or returns "" if it's not set
 func URLUser(input string) (string, error) {
 	u, err := url.Parse(input)