Configure nginx for webserver-alkane

Summary:
Promote the current data for webserver-legacy to webserver-alkane role.

Files in roles/webserver-alkane/nginx/files/includes/ are provisioned
everywhere, like on webserver-legacy.

Vhosts can be provisioned to www-alkane (web-001) or www-dev (windriver)
by declaring them in a file in the relevant pillar/paas/alkane/ folder.
That's a breaking change from webserver-legacy. This change takes care
to dispatch the vhosts to the relevant server.

Ref T1828

Test Plan: Deploy to WindRiver

Reviewers: dereckson, ledesillusionniste

Reviewed By: dereckson

Maniphest Tasks: T1828

Differential Revision: https://devcentral.nasqueron.org/D3160