Maintain consistency of nodes field

There are four scenarios in which the nodes field can change after the
disk's creation; when moving, replacing, converting or recreating disks.
For each scenario, update the nodes field accordingly.
Update cfgupgrade tool

Create an upgrade function that adds a nodes field to each disk that is
attached to an instance. Also, add a downgrade function that removes the
nodes field and brings the config to the default state for 2.12.
Update various tests regarding disk creation

Update the tests that deal with disk creation and check if the nodes
field is populated correctly when creating a disk.