Avoid name conflicts in nginx unit built on concrete webserver roles

Summary:
The saas-mediawiki role is for example used with webserver-legacy on Ysul.
Both used the vhosts and includes folder as ID. That creates a conflict
if the highstate is applied, instead of the current practice to apply roles
one by one.

Test Plan: Deploy on Ysul and Dwellers

Reviewers: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3026