provider/aws: Change AWS ssm_maintenance_window Read func (#14665)

Fixes: #14653

I was originally calling the wrong API method and only some of the
values were being persisted to state. By changing the API method, we can
now get all of the values and therefore can detech manual drift

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSSSMMaintenanceWindow_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/19 16:56:27 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSSSMMaintenanceWindow_ -timeout 120m
=== RUN   TestAccAWSSSMMaintenanceWindow_basic
--- PASS: TestAccAWSSSMMaintenanceWindow_basic (41.39s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	41.419s
```

builtin/providers/aws/resource_aws_ssm_maintenance_window.go

@@ -68,7 +68,6 @@ func resourceAwsSsmMaintenanceWindowCreate(d *schema.ResourceData, meta interfac
 	}
 
 	d.SetId(*resp.WindowId)
-
 	return resourceAwsSsmMaintenanceWindowRead(d, meta)
 }
 
@@ -114,38 +113,21 @@ func resourceAwsSsmMaintenanceWindowUpdate(d *schema.ResourceData, meta interfac
 func resourceAwsSsmMaintenanceWindowRead(d *schema.ResourceData, meta interface{}) error {
 	ssmconn := meta.(*AWSClient).ssmconn
 
-	params := &ssm.DescribeMaintenanceWindowsInput{
-		Filters: []*ssm.MaintenanceWindowFilter{
-			{
-				Key:    aws.String("Name"),
-				Values: []*string{aws.String(d.Get("name").(string))},
-			},
-		},
+	params := &ssm.GetMaintenanceWindowInput{
+		WindowId: aws.String(d.Id()),
 	}
 
-	resp, err := ssmconn.DescribeMaintenanceWindows(params)
+	resp, err := ssmconn.GetMaintenanceWindow(params)
 	if err != nil {
 		return err
 	}
 
-	found := false
-
-	for _, window := range resp.WindowIdentities {
-		if *window.WindowId == d.Id() {
-			found = true
-
-			d.Set("name", window.Name)
-			d.Set("cutoff", window.Cutoff)
-			d.Set("duration", window.Duration)
-			d.Set("enabled", window.Enabled)
-		}
-	}
-
-	if !found {
-		log.Printf("[INFO] Cannot find the SSM Maintenance Window %q. Removing from state", d.Get("name").(string))
-		d.SetId("")
-		return nil
-	}
+	d.Set("name", resp.Name)
+	d.Set("cutoff", resp.Cutoff)
+	d.Set("duration", resp.Duration)
+	d.Set("enabled", resp.Enabled)
+	d.Set("allow_unassociated_targets", resp.AllowUnassociatedTargets)
+	d.Set("schedule", resp.Schedule)
 
 	return nil
 }