Support failover for forthcoming instances

...by simply changing the configuration and not doing
anything in the real world.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Niklas Hambuechen <niklash@google.com>
Move out the updating nodes in disk configuration

...to a separate method. In this way it can be reused
for both replacing disks of real, and of forthcoming
instances.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Niklas Hambuechen <niklash@google.com>
Support replacing disks for forthcoming instances

...by just modifying the configuration and not changing
anything in the real world.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Niklas Hambuechen <niklash@google.com>