AppManager: Support exposing cross-cluster ports

Change-Id: I4bdb3573209935f6777656ec2f3481e79d84a9c9
diff --git a/core/installer/app.go b/core/installer/app.go
index 948e17e..e57fee8 100644
--- a/core/installer/app.go
+++ b/core/installer/app.go
@@ -95,6 +95,7 @@
 }
 
 type PortForward struct {
+	Cluster     string `json:"cluster,omitempty"`
 	Allocator   string `json:"allocator"`
 	ReserveAddr string `json:"reservator"`
 	RemoveAddr  string `json:"deallocator"`