This commit address a problem in static mode, if the hostname change,
`host.name` value with change too. And as a result:
results['dns_config_result'][host.name]
will be initialized.
We now record the initial hostname first, and uses it as the key for the
results.
This commit also ensures hostname and search domain change are detected if
`instance.dnsConfig.dhcp` is true.
Finally, if we target a cluster, the `changed` result will depend on the
number of nodes.
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