chore: update config sample to explain config for no worker nodes

Signed-off-by: Devin Buhl <devin@buhl.casa>

bootstrap/vars/config.sample.yaml

@@ -63,7 +63,7 @@ bootstrap_nodes:
     #   username: # ssh username of the master node
     #   password: # password of ssh username for the master node
     # ...
-  worker:
+  worker: # set to [] or omit if no workers are needed
     # - name:     # name of the worker node (must match [a-z0-9-]+)
     #   address:  # ip address of the worker node
     #   username: # ssh username of the worker node