Merge pull request #41220 from errordeveloper/fix-hostname-preflight-check

Automatic merge from submit-queue (batch tested with PRs 41223, 40892, 41220, 41207, 41242)

kubeadm: preflight should only warn about unresolvable hostnames

**What this PR does / why we need it**:

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 requirement
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.

**Special notes for your reviewer**: @dmmcquay @luxas PTAL 👍 

**Release note**:

```release-note
NONE
```

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