host-metrics: correct deploy create command shape

`deploy create` requires `--stack` (deploy.py:70) and lives in the
`deploy` group, not `deployment`. `laconic-so deployment create ...`
does not exist as a subcommand.

stack_orchestrator/data/stacks/host-metrics/README.md

@@ -60,10 +60,17 @@ metrics.
 ### Create and start
 
 ```bash
-laconic-so deployment create --spec-file spec-host-metrics.yml --deployment-dir ./deployment-host-metrics
+laconic-so --stack host-metrics deploy create \
+    --spec-file spec-host-metrics.yml \
+    --deployment-dir ./deployment-host-metrics
 laconic-so deployment --dir ./deployment-host-metrics start
 ```
 
+`deploy create` builds the deployment dir from the spec; `deployment start`
+brings the containers up. The `--stack` option is required for `deploy`
+subcommands but rejected on `deployment` subcommands (the deployment dir
+already knows its stack).
+
 ### Verify
 
 ```bash