Replace removed yaml module

In elastic#16586 the module include was
removed. This causes failures in the script when the module is referenced. This
commit re-enables the include for the yaml module.

docker/data/logstash/env2yaml/env2yaml.go

@@ -17,6 +17,7 @@ package main
 import (
 	"errors"
 	"fmt"
+	"gopkg.in/yaml.v2"
 	"io/ioutil"
 	"log"
 	"os"