Add unique constraint to instance_uuid

Add a unique constraint to the instance_uuid column of the nodes table
and a migration script. The old node_instance_uuid index was dropped in
favor of the new unique index.

Partial-Bug: #1326364
Change-Id: I005d3b3f252b18c506bab3297b1a5e3c7e7534d2
Merge "Add unique constraint to instance_uuid"