Add support for dotenv / env-based secrets

[skluck]

Configuration secrets should be passed in the environment instead of stored in the DI container.

Symfony has supported ENV-based runtime parameters since 3.2 - http://symfony.com/blog/new-in-symfony-3-2-runtime-environment-variables

A dotenv component was introduced in 3.3 - http://symfony.com/blog/new-in-symfony-3-3-dotenv-component.

While it is best for any infrastructure of hal to provide these values in the env, we can also store these values in env.yml files for environments such as dev.

• [x] Hal UI
• [x] Hal Agent

[skluck]

This was finished recently in august.