Refactor GetForInstance so it doesn't depends on storage

docker/factory.go

@@ -1,6 +1,8 @@
 package docker
 
+import "github.com/play-with-docker/play-with-docker/pwd/types"
+
 type FactoryApi interface {
 	GetForSession(sessionId string) (DockerApi, error)
-	GetForInstance(sessionId, instanceName string) (DockerApi, error)
+	GetForInstance(instance *types.Instance) (DockerApi, error)
 }