Enhancement: Add support for listing vault folders

[invartam]

Introduction

In vault, secrets can be organized under a folder hierarchy on KVv1 and KVv2 secret engines. In some case we need to list folders that are in a folder in order to loop on them and query secrets inside them. We can do it using vault-cli and curl requests as written in vault documentation for both KVv1 and KVv2.

Request

We need consul-template to support the listing of vault folders in secret engines KVv1 and KVv2 like we can already do for secrets and consul objects.

Vault implementations

https://www.vaultproject.io/api/secret/kv/kv-v1.html#list-secrets

https://www.vaultproject.io/api/secret/kv/kv-v2.html#list-secrets