Fix non-unique tftp dir instance_uuid

The driver 'instance_uuid' property was used to determine
the tftp directory name for the instance's images.
Since there is no guarantee that the intance_uuid will be unique,
we will use node uuid.

Change-Id: Ie8422b39eb6007b833d09be1855874a82dcb17d4
Merge "Fix non-unique tftp dir instance_uuid"