Support for attachments in vanilla plugin (#455)

Signed-off-by: David Chung <david.chung@docker.com>

Author: David Chung

docs/cattle.json

@@ -20,7 +20,8 @@
         "Tags": {
           "tier": "web",
           "project": "infrakit"
-        }
+        },
+        "Attachments" :  [{"ID":"attachid", "Type": "attachtype"}]
       }
     }
   }

docs/cattle2.json

@@ -20,7 +20,8 @@
         "Tags": {
           "tier": "web",
           "project": "infrakit"
-        }
+        },
+        "Attachments" :  [{"ID":"attachid", "Type": "attachtype"}]
       }
     }
   }

docs/tutorial.md

@@ -95,7 +95,8 @@ Putting everything together, we have the configuration to give to the default Gr
         "Tags": {
           "tier": "web",
           "project": "infrakit"
-        }
+        },
+        "Attachments" :  [{"ID":"attachid", "Type": "attachtype"}]
       }
     }
   }
@@ -192,7 +193,8 @@ Now let's update the configuration by changing the size of the group and a prope
         "Tags": {
           "tier": "web",
           "project": "infrakit"
-        }
+        },
+        "Attachments" :  [{"ID":"attachid", "Type": "attachtype"}]
       }
     }
   }

examples/flavor/vanilla/flavor.go

@@ -16,6 +16,9 @@ type Spec struct {
 
 	// Tags
 	Tags map[string]string
+
+	// Attachments are instructions for external entities that should be attached to the instance.
+	Attachments []instance.Attachment
 }
 
 // NewPlugin creates a Flavor plugin that doesn't do very much. It assumes instances are
@@ -69,5 +72,10 @@ func (f vanillaFlavor) Prepare(flavor *types.Any,
 		}
 		instance.Tags[k] = v
 	}
+
+	// Attachements
+	for _, a := range s.Attachments {
+		instance.Attachments = append(instance.Attachments, a)
+	}
 	return instance, nil
 }