ClientConfig should not default to http://localhost:8080

This changes clientcmd to skip the default cluster, but preserves the
behavior in kubectl. This prevents the possibility of an administrator
misconfiguration in kubelet or other server component from allowing a
third party who can bind to 8080 on that host from potentially
impersonating an API server and gaining root access.

pkg/client/unversioned/clientcmd/overrides.go

@@ -27,10 +27,12 @@ import (
 // ConfigOverrides holds values that should override whatever information is pulled from the actual Config object.  You can't
 // simply use an actual Config object, because Configs hold maps, but overrides are restricted to "at most one"
 type ConfigOverrides struct {
-	AuthInfo       clientcmdapi.AuthInfo
-	ClusterInfo    clientcmdapi.Cluster
-	Context        clientcmdapi.Context
-	CurrentContext string
+	AuthInfo clientcmdapi.AuthInfo
+	// ClusterDefaults are applied before the configured cluster info is loaded.
+	ClusterDefaults clientcmdapi.Cluster
+	ClusterInfo     clientcmdapi.Cluster
+	Context         clientcmdapi.Context
+	CurrentContext  string
 }
 
 // ConfigOverrideFlags holds the flag names to be used for binding command line flags.  Notice that this structure tightly