Fix non-unique pxe driver 'instance_name'

The driver 'instance_name' property was used to determine
the image directory name for the instance's image storage.
Since there is no guarantee that the intance_name will be unique,
we will use node uuid.

Change-Id: I781424215bdeb5372ceef2d6f11737e8f003054d
Closes-Bug: #1236536
Merge "Fix non-unique pxe driver 'instance_name'"