Support `node[:dnsmasq][:enable_hostsfile]`

In some environments, dnsmasq might be wanted but not the addition of
items to `/etc/hosts` based on dnsmasq.  Support such use-cases by
letting `node[:dnsmasq][:enable_hostsfile]` be set false.

This is a variation of something carried locally, ported forward to the current dnsmasq code-base.