Migrate directories unit to webserver-alkane role

Summary:
This pattern works well: with the concept of one domain = one user
for static sites, it creates /var/wwwroot/<domain.tlD>.

This change is so adopted for Alkane.

Ref T1828

Test Plan: Deploy on web-001

Reviewers: dereckson

Maniphest Tasks: T1828

Differential Revision: https://devcentral.nasqueron.org/D3041