Daemon/host/role accessories

Allow the hosts for accessories to be specified by host or role, or on
all app hosts by setting `daemon: true`.

```
  # Single host
  mysql:
    host: 1.1.1.1
  # Multiple hosts
  redis:
    hosts:
      - 1.1.1.1
      - 1.1.1.2
  # By role
  monitoring:
    roles:
      - web
      - jobs
```

lib/mrsk/commander.rb

@@ -59,7 +59,7 @@ class Mrsk::Commander
   end
 
   def accessory_hosts
-    specific_hosts || config.accessories.collect(&:host)
+    specific_hosts || config.accessories.flat_map(&:hosts)
   end
 
   def accessory_names