Provision .env credentials per environment

Summary:
The roles "webserver-alkane-prod" and "webserver-alkane-dev" allow to distribute
credentials per environment, like for "paas-docker-".

Documentation is migrated to Alkane:
https://agora.nasqueron.org/Operations_grimoire/Alkane#Provision_credentials_with_.env

Ref T1803 (prod)
Ref T1897 (dev)

Test Plan: Deploy to WindRiver

Reviewers: dereckson

Maniphest Tasks: T1897, T1803

Differential Revision: https://devcentral.nasqueron.org/D3212