kubeadm: preflight should only warn about unresolvable hostnames

This is quite often the case on AWS, and we really don't care if
the hostname is resolvable or not. It's not an easy problem
to ask user to fix, and there is no functional penalty at the
Kubernetes level, also it's possible that users fixes their host
resolution eventually, we don't have to make them do so.

cmd/kubeadm/app/preflight/checks.go

@@ -239,18 +239,19 @@ type HostnameCheck struct{}
 
 func (hc HostnameCheck) Check() (warnings, errors []error) {
 	errors = []error{}
+	warnings = []error{}
 	hostname := node.GetHostname("")
 	for _, msg := range validation.ValidateNodeName(hostname, false) {
 		errors = append(errors, fmt.Errorf("hostname \"%s\" %s", hostname, msg))
 	}
 	addr, err := net.LookupHost(hostname)
 	if addr == nil {
-		errors = append(errors, fmt.Errorf("hostname \"%s\" could not be reached", hostname))
+		warnings = append(warnings, fmt.Errorf("hostname \"%s\" could not be reached", hostname))
 	}
 	if err != nil {
-		errors = append(errors, fmt.Errorf("hostname \"%s\" %s", hostname, err))
+		warnings = append(warnings, fmt.Errorf("hostname \"%s\" %s", hostname, err))
 	}
-	return nil, errors
+	return warnings, errors
 }
 
 // HTTPProxyCheck checks if https connection to specific host is going