GRES: Node by node upgrade

By default, Junos OS Evolved upgrades the entire cluster during a maintenance window, including both Routing Engines and all Flexible PIC Concentrators (FPCs). With this feature, you can upgrade one Routing Engine at a time by temporarily disabling Routing Engine synchronization, upgrading the hardware backup Routing Engine first, switching over to it, then upgrading the new backup Routing Engine and re-enabling Routing Engine synchronization.