Consolidate web content units into webserver-content role

Summary:
Roles

The webserver-content role will receive all instructions
to provision /var/wwwroot folders.

The organization follows the one defined in the first commits
for webserver-legacy/, with folders and subfolders according
the FQDN.

States

The eglide-website unit contained also nginx configuration,
moved temporarily to the shellserver/web-hosting role.
In the short to medium term, it will be consolidated into
a more generic webserver-core.

Pillar

The salt master knows what to deploy on what server through
the web_content_sls pillar entry.

The webserver pillar entries are now available to each server.

Refs

The commits a0d4bf923805 and 23815a361565 have prepared the groundwork.

Ref T1325

Test Plan:
First, check Eglide and Ysul got the correct ones:

• salt-call --local node.filter_by_role web_content_sls
• salt eglide node.filter_by_role web_content_sls

Then, ensure salt * state.apply webserver-content is no-op.

Units tests added for node.has_web_content

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1325

Differential Revision: https://devcentral.nasqueron.org/D1259