plan: inform user if a deployment will be created

Just a proof of concept because the plan output when deployments are
involved is a bit non-obvious. When updating a job with count=3, the
plan will only show the first step of the deployment (as determined by
update.max_parallel):

```
+/- Job: "sleeper"
+/- Task Group: "sleeper" (1 create/destroy update, 2 ignore)
  +/- Task: "sleeper" (forces create/destroy update)
      +/- Config {
```

This hack plumbs through whether a deployment was attached to the plan.
Obviously the output and field names need improving. For backward
compatibility we probably shouldn't output _anything_ if there isn't a
deployment, but this PoC includes a message for either case for ease of
testing/demonstration.

api/jobs.go

@@ -1525,6 +1525,7 @@ type JobPlanResponse struct {
 	CreatedEvals       []*Evaluation
 	Diff               *JobDiff
 	Annotations        *PlanAnnotations
+	CauseDeployment    bool
 	FailedTGAllocs     map[string]*AllocationMetric
 	NextPeriodicLaunch time.Time
 

command/job_plan.go

@@ -310,6 +310,12 @@ func (c *JobPlanCommand) outputPlannedJob(job *api.Job, resp *api.JobPlanRespons
 			c.Colorize().Color(strings.TrimSpace(formatJobDiff(resp.Diff, verbose)))))
 	}
 
+	if resp.CauseDeployment {
+		c.Ui.Output("deployment time!")
+	} else {
+		c.Ui.Output("no deployment")
+	}
+
 	// Print the scheduler dry-run output
 	c.Ui.Output(c.Colorize().Color("[bold]Scheduler dry-run:[reset]"))
 	c.Ui.Output(c.Colorize().Color(formatDryRun(resp, job)))

nomad/job_endpoint.go

@@ -1984,6 +1984,7 @@ func (j *Job) Plan(args *structs.JobPlanRequest, reply *structs.JobPlanResponse)
 	reply.FailedTGAllocs = updatedEval.FailedTGAllocs
 	reply.JobModifyIndex = index
 	reply.Annotations = annotations
+	reply.CauseDeployment = planner.Plans[0].Deployment != nil
 	reply.CreatedEvals = planner.CreateEvals
 	reply.Index = index
 	return nil

nomad/structs/structs.go

@@ -1689,6 +1689,10 @@ type JobPlanResponse struct {
 	// causes an in-place update or create/destroy
 	Diff *JobDiff
 
+	// CauseDeployment is true if the plan causes a deployment to be created or
+	// modified.
+	CauseDeployment bool
+
 	// NextPeriodicLaunch is the time duration till the job would be launched if
 	// submitted.
 	NextPeriodicLaunch time.Time