Consolidate nginx installation and common config files

Summary:
Both shellserver and webserver-legacy roles contains states
useful for every server with nginx. Those states are moved to
the webserver-core role.

Let's encrypt certbot client doesn't need to be installed for
Docker engines, as it runs then as a container.

Test Plan:

• salt equatower state.apply roles/webserver-core (Docker, should provision)
• salt eglide state.apply roles/webserver-core test=True (non Docker, should be no op)

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1415