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
```

test/fixtures/deploy_simple.yml

@@ -0,0 +1,8 @@
+service: app
+image: dhh/app
+servers:
+  - "1.1.1.1"
+  - "1.1.1.2"
+registry:
+  username: user
+  password: pw

test/fixtures/deploy_with_accessories.yml

@@ -1,8 +1,12 @@
 service: app
 image: dhh/app
 servers:
-  - 1.1.1.1
-  - 1.1.1.2
+  web:
+    - "1.1.1.1"
+    - "1.1.1.2"
+  workers:
+    - "1.1.1.3"
+    - "1.1.1.4"
 registry:
   username: user
   password: pw
@@ -23,7 +27,8 @@ accessories:
       - data:/var/lib/mysql
   redis:
     image: redis:latest
-    host: 1.1.1.4
+    roles:
+      - web
     port: 6379
     directories:
       - data:/data