We used to unconditionally add the name and IP address(es) for the
node itself to /etc/hosts when managing it using networking::hostsfile.
However, that is a bit unflexible, as it depends on facter(1) being
correct in guessing what the primary IPv4 and/or IPv6 address for the
node is.  That's not always the case.

Here, this is changed, so networking::hostsfile does not add the node
itself to /etc/hosts.  Instead, users must now include the new class
'networking::hostsfile::node_self' to get the old behaviour.

This change is strictly speaking not backwards compatible, but since
we never got around to publishing the previous version, no one will
suffer for the change.