Since you want to copy the current values into hostDnsConfig to insure that all required parameters are present, you might just initialize as

    @host_dns ||= {}.merge host.config.network.dnsConfig

There will be no extra properties, since in this particular resource the 'current' value from the API is the same type (HostDnsConfig) as the data object to be sent to the API to change the configuration. 

Then you can eliminate the host_dns[foo_sym ] ||= foo lines from flush