Revert removal of EndpointName hooks (#2319)

* Revert "Remove dead EndpointName code"
    This reverts commit 8025da5a8d.
* Fix docstrings based on proper understanding of use

pkg/cluster/managed.go

@@ -6,9 +6,11 @@ package cluster
 import (
 	"context"
 	"net/http"
+	"strings"
 	"time"
 
 	"github.com/rancher/k3s/pkg/cluster/managed"
+	"github.com/rancher/kine/pkg/endpoint"
 	"github.com/sirupsen/logrus"
 )
 
@@ -62,11 +64,21 @@ func (c *Cluster) initClusterDB(ctx context.Context, handler http.Handler) (http
 		return handler, nil
 	}
 
+	if !strings.HasPrefix(c.config.Datastore.Endpoint, c.managedDB.EndpointName()+"://") {
+		c.config.Datastore = endpoint.Config{
+			Endpoint: c.managedDB.EndpointName(),
+		}
+	}
+
 	return c.managedDB.Register(ctx, c.config, handler)
 }
 
-// assignManagedDriver checks to see if any managed databases are already configured or should be created/joined.
-// If a driver has been initialized it is used, otherwise we create or join a cluster using the default driver.
+// assignManagedDriver assigns a driver based on a number of different configuration variables.
+// If a driver has been initialized it is used.
+// If the configured endpoint matches the name of a driver, that driver is used.
+// If no specific endpoint has been requested and creating or joining has been requested,
+// we use the default driver.
+// If none of the above are true, no managed driver is assigned.
 func (c *Cluster) assignManagedDriver(ctx context.Context) error {
 	// Check all managed drivers for an initialized database on disk; use one if found
 	for _, driver := range managed.Registered() {
@@ -78,6 +90,16 @@ func (c *Cluster) assignManagedDriver(ctx context.Context) error {
 		}
 	}
 
+	// This is needed to allow downstreams to override driver selection logic by
+	// setting ServerConfig.Datastore.Endpoint such that it will match a driver's EndpointName
+	endpointType := strings.SplitN(c.config.Datastore.Endpoint, ":", 2)[0]
+	for _, driver := range managed.Registered() {
+		if endpointType == driver.EndpointName() {
+			c.managedDB = driver
+			return nil
+		}
+	}
+
 	// If we have been asked to initialize or join a cluster, do so using the default managed database.
 	if c.config.Datastore.Endpoint == "" && (c.config.ClusterInit || (c.config.Token != "" && c.config.JoinURL != "")) {
 		for _, driver := range managed.Registered() {