An issue was found in 1.5.6 that calls for an early next release to fix that. Find the details here if you run into problems with FTP connections on kernel updates or storage nodes in 1.5.6: https://github.com/FOGProject/fogproject/issues/311
I’m not sure I understand your problem just yet.
Let me see if I understand it using my words.
"I have fog servers and storage nodes with 2 network interfaces. One interface is dedicated to imaging on an isolated imaging network at each location. I want to setup a master->storage node replication between the fog server, but I want the replication to happen over the second network interface and not the isolated imaging network. "
Did I describe the situation?