Use 'ipv4' and 'ipv6' elements in hostsfile::hostentries.

As the parse_host_mac_ip_list() function now recommends not using the
'ip' element of its return value, instead using the 'ipv4' and 'ipv6'
elements, we should of course follow that ourselves.  Specifically,
this affected the networking::hostsfile::hostentries definition, which
used the 'ip' element.  We here change it to use the 'ipv4' and 'ipv6'
elements if present, but for backward compatibility it will still use
the 'ip' element if both 'ipv4' and 'ipv6' elements are missing.

The networking::hostsfile::node_self definition also needed to be
updated, but this does not affect users.