Add an optional route to managed nodes for connections to a shared build node

[bo-quan]

In a shared build environment, we provision build vms on a centralized server to manage multiple pods using snaps-boot. The build vm has a floating ip, as well as access to the management network; however, the managed nodes would try to reach the build vm's floating ip via their public network. We should add an option to add a route to the managed nodes to communicate with the build vm via a specific NIC (e.g., the NIC on the management network instead of the NIC on the public network by default.)