nodeup: create kubeconfig under admin or root

While the admin account is created on stock debian images, it isn't on
all of them.  Check admin first, then check root, and don't treat it as
an error if neither is found - this is only a convenience.

nodeup/pkg/model/kubectl.go

@@ -74,29 +74,64 @@ func (b *KubectlBuilder) Build(c *fi.ModelBuilderContext) error {
 		}
 		c.AddTask(t)
 
-		switch b.Distribution {
-		case distros.DistributionJessie, distros.DistributionDebian9:
+		adminUser, adminGroup, err := b.findKubeconfigUser()
+		if err != nil {
+			return err
+		}
+
+		if adminUser != "" {
 			c.AddTask(&nodetasks.File{
 				Path:  "/home/admin/.kube/",
 				Type:  nodetasks.FileType_Directory,
 				Mode:  s("0700"),
-				Owner: s("admin"),
-				Group: s("admin"),
+				Owner: s(adminUser),
+				Group: s(adminGroup),
 			})
 
 			c.AddTask(&nodetasks.File{
 				Path:     "/home/admin/.kube/config",
 				Contents: fi.NewStringResource(kubeconfig),
 				Type:     nodetasks.FileType_File,
 				Mode:     s("0400"),
-				Owner:    s("admin"),
-				Group:    s("admin"),
+				Owner:    s(adminUser),
+				Group:    s(adminGroup),
 			})
-
-		default:
-			glog.Warningf("Unknown distro; won't write kubeconfig to homedir %s", b.Distribution)
 		}
 	}
 
 	return nil
 }
+
+// findKubeconfigUser finds the default user for whom we should create a kubeconfig
+func (b *KubectlBuilder) findKubeconfigUser() (string, string, error) {
+	var users []string
+	switch b.Distribution {
+	case distros.DistributionJessie, distros.DistributionDebian9:
+		users = []string{"admin", "root"}
+	default:
+		glog.Warningf("Unknown distro; won't write kubeconfig to homedir %s", b.Distribution)
+		return "", "", nil
+	}
+
+	for _, s := range users {
+		user, err := fi.LookupUser(s)
+		if err != nil {
+			glog.Warningf("error looking up user %q: %v", s, err)
+			continue
+		}
+		if user == nil {
+			continue
+		}
+		group, err := fi.LookupGroupById(user.Gid)
+		if err != nil {
+			glog.Warningf("unable to find group %d for user %q", user.Gid, s)
+			continue
+		}
+		if group == nil {
+			continue
+		}
+		return user.Name, group.Name, nil
+	}
+
+	return "", "", nil
+}