Parameterize the deployment of server web site

Summary:
Servers pages should be provided at standard locations, like
/var/wwwroot/nasqueron.org/ysul.

Currently, eglide and dwellers store at the default nginx
directory their respective homepages.

As a first step in this direction, a node.get_wwroot helps
to determine from a server hostname the directory to use.

Another issue is nginx is installed under www-data on Debian,
while on FreeBSD it's installed as www. There is also a custom
group declared in webserver-legacy account unit.

Test Plan: Ensure it's no-op on Eglide

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1253