Don't install software in the core role

Summary:
Core is about configure a server, not about installing stuff.

We want to avoid to install software on containers OS.

Fixes T1259.

Test Plan:
Ensure this is no-op:
salt eglide state.apply roles/webserver-core/letsencrypt

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1259

Differential Revision: https://devcentral.nasqueron.org/D1064