cli: docker service|node|stack ps instead of tasks

Rather than conflict with the unexposed task model, change the names of
the object-oriented task display to `docker <object> ps`. The command
works identically to `docker service tasks`. This change is superficial.

This provides a more sensical docker experience while not trampling on
the task model that may be introduced as a top-level command at a later
date.

The following is an example of the display using `docker service ps`
with a service named `condescending_cori`:

```
$ docker service ps condescending_cori
ID                         NAME                  SERVICE             IMAGE   LAST STATE              DESIRED STATE  NODE
e2cd9vqb62qjk38lw65uoffd2  condescending_cori.1  condescending_cori  alpine  Running 13 minutes ago  Running        6c6d232a5d0e
```

The following shows the output for the node on which the command is
running:

```console
$ docker node ps self
ID                         NAME                  SERVICE             IMAGE   LAST STATE              DESIRED STATE  NODE
b1tpbi43k1ibevg2e94bmqo0s  mad_kalam.1           mad_kalam           apline  Accepted 2 seconds ago  Accepted       6c6d232a5d0e
e2cd9vqb62qjk38lw65uoffd2  condescending_cori.1  condescending_cori  alpine  Running 12 minutes ago  Running        6c6d232a5d0e
4x609m5o0qyn0kgpzvf0ad8x5  furious_davinci.1     furious_davinci     redis   Running 32 minutes ago  Running        6c6d232a5d0e
```

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/reference/commandline/node_update.md

@@ -59,6 +59,6 @@ metadata](../../userguide/labels-custom-metadata.md).
 ## Related information
 
 * [node inspect](node_inspect.md)
-* [node tasks](node_tasks.md)
+* [node ps](node_ps.md)
 * [node ls](node_ls.md)
 * [node rm](node_rm.md)