Read credentials and config from environment

Summary:
On Nasqueron Alkane PaaS, secrets are provisioned through a .env file
while other config is available in php-fpm environment.

References:

• https://12factor.net/config
• https://devcentral.nasqueron.org/D3068
• https://devcentral.nasqueron.org/D3073

Test Plan:

• deploy on web-001, ensure we've credentials
• check it's no-op locally

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://code.zed.dereckson.be/D20