Add support for Kubernetes API Authn Webhook

Allow multiple authn strategies to be defined, including new 'webhook'
strategy. Webhook strategy configuration contains the contents of the
authentication webhook file as well as the cache timeout period.

This change allows a Kubernetes API Auth service to authenticate
user requests without proxying through the Rancher server.

cluster/file-deployer.go

@@ -0,0 +1,60 @@
+package cluster
+
+import (
+	"context"
+	"fmt"
+	"path"
+
+	"github.com/docker/docker/api/types/container"
+	"github.com/rancher/rke/docker"
+	"github.com/rancher/rke/hosts"
+	"github.com/rancher/rke/log"
+	"github.com/rancher/types/apis/management.cattle.io/v3"
+	"github.com/sirupsen/logrus"
+)
+
+const (
+	ContainerName = "file-deployer"
+	ServiceName   = "file-deploy"
+	ConfigEnv     = "FILE_DEPLOY"
+)
+
+func deployFile(ctx context.Context, uniqueHosts []*hosts.Host, alpineImage string, prsMap map[string]v3.PrivateRegistry, fileName, fileContents string) error {
+	for _, host := range uniqueHosts {
+		log.Infof(ctx, "[%s] Deploying file '%s' to node [%s]", ServiceName, fileName, host.Address)
+		if err := doDeployFile(ctx, host, fileName, fileContents, alpineImage, prsMap); err != nil {
+			return fmt.Errorf("Failed to deploy file '%s' on node [%s]: %v", host.Address, fileName, err)
+		}
+	}
+	return nil
+}
+
+func doDeployFile(ctx context.Context, host *hosts.Host, fileName, fileContents, alpineImage string, prsMap map[string]v3.PrivateRegistry) error {
+	// remove existing container. Only way it's still here is if previous deployment failed
+	if err := docker.DoRemoveContainer(ctx, host.DClient, ContainerName, host.Address); err != nil {
+		return err
+	}
+	containerEnv := []string{ConfigEnv + "=" + fileContents}
+	imageCfg := &container.Config{
+		Image: alpineImage,
+		Cmd: []string{
+			"sh",
+			"-c",
+			fmt.Sprintf("t=$(mktemp); echo -e \"$%s\" > $t && mv $t %s && chmod 644 %s", ConfigEnv, fileName, fileName),
+		},
+		Env: containerEnv,
+	}
+	hostCfg := &container.HostConfig{
+		Binds: []string{
+			fmt.Sprintf("%s:/etc/kubernetes:z", path.Join(host.PrefixPath, "/etc/kubernetes")),
+		},
+	}
+	if err := docker.DoRunContainer(ctx, host.DClient, imageCfg, hostCfg, ContainerName, host.Address, ServiceName, prsMap); err != nil {
+		return err
+	}
+	if err := docker.DoRemoveContainer(ctx, host.DClient, ContainerName, host.Address); err != nil {
+		return err
+	}
+	logrus.Debugf("[%s] Successfully deployed file '%s' on node [%s]", ServiceName, fileName, host.Address)
+	return nil
+}