To deploy a Varnish cache, a specific role to assign
to front-end servers 'webserver-varnish' is provided.

The Varnish package is called Varnish everywhere,
excepted currently on FreeBSD where it's varnish5.
As such, this change adds the package name to the map.

Ref. T1243.

When the Salt ran on Ysul:
Oct 9 23:52:35 ysul pkg: varnish5-5.2.0 installed