Added support for env_vars

**What does this PR do?**
It adds support for rbenv_vars plugin, so you can add `env_vars` to your json payload, and it will create the `.rbenv_vars` file in the `YOURAPP/shared` directory.


**Example json payload**
```ruby
  json_payload = {
    "authorization" => {...},
    "mysql" => {...},
    "postgresql" => {...},
    "active_applications" => {
      "sample_app" => {
        ...
        "env_vars" => {
          "hello"=> "world",
          "rails_secret"=> "such_secret"
        },...

```