Consolidate nginx configuration into webserver-core role

Summary:
Several roles configures nginx:

• paas-docker
• shellserver (Eglide)
• webserver-legacy

Currently, each of them defines its own nginx configuration.

This change allows to configure nginx in two layers:

• a common one in webserver-core role
• specific vhosts and includes in the specific role

Ref T1828

Test Plan: Deploy on Eglide, Dwellers, Ysul then web-001

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1828

Differential Revision: https://devcentral.nasqueron.org/D2999