PXE instance_name is no longer mandatory

With the use of node.uuid as the property to build the directory
name for the instance images, instance_name is not longer needed
to validate the pxe driver info.

Change-Id: I1839bcd5bf0ca0f3d6e62d5760ede65c1ec3e8e8
Merge "PXE instance_name is no longer mandatory"