cli: update default redis and use nomad service discovery

Closes #12927
Closes #12958

This PR updates the version of redis used in our examples from 3.2 to 7.
The old version is very not supported anymore, and we should be setting
a good example by using a supported version.

The long-form example job is now fixed so that the service stanza uses
nomad as the service discovery provider, and so now the job runs without
a requirement of having Consul running and configured.

client/allocrunner/taskrunner/task_runner_test.go

@@ -681,7 +681,7 @@ func TestTaskRunner_TaskEnv_Image(t *testing.T) {
 	task := alloc.Job.TaskGroups[0].Tasks[0]
 	task.Driver = "docker"
 	task.Config = map[string]interface{}{
-		"image":        "redis:3.2-alpine",
+		"image":        "redis:7-alpine",
 		"network_mode": "none",
 		"command":      "sh",
 		"args": []string{"-c", "echo $NOMAD_ALLOC_DIR; " +