Fix preflight check failure on node join.

We recently improved this to tolerate existence of things like
/etc/kubernetes/manifests/ as an empty dir, but forgot to do so for the
join pre-flight checks where it is also failing.

Instead ensure only the sub-directories and files we need are available.

Author: dgoodwin

Diff for: cmd/kubeadm/app/preflight/checks.go

@@ -258,8 +258,9 @@ func RunJoinNodeChecks(cfg *kubeadmapi.NodeConfiguration) error {
 		PortOpenCheck{port: 10250},
 		HttpProxyCheck{Proto: "https", Host: cfg.MasterAddresses[0], Port: int(cfg.APIPort)},
 		HttpProxyCheck{Proto: "http", Host: cfg.MasterAddresses[0], Port: int(cfg.DiscoveryPort)},
-		DirAvailableCheck{Path: "/etc/kubernetes"},
+		DirAvailableCheck{Path: "/etc/kubernetes/manifests"},
 		DirAvailableCheck{Path: "/var/lib/kubelet"},
+		FileAvailableCheck{Path: "/etc/kubernetes/kubelet.conf"},
 		InPathCheck{executable: "ebtables", mandatory: true},
 		InPathCheck{executable: "ethtool", mandatory: true},
 		InPathCheck{executable: "ip", mandatory: true},