Maintain consistency of nodes list

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

Create an upgrade function that adds a nodes list to each disk that is
attached to an instance. Also, add a downgrade function that removes the
nodes list 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
list is populated correctly when creating a disk.