PXE driver should not accept empty kernel UUID.

Baremetal PXE driver was aborting only if the kernel and ramdisk
specified in image metadata were None, but not if they were another
non-true value, such as "". Now, exception is raised if they are any
non-true value.

Fix bug 1100589.

Change-Id: I9783a14cc242e1b9db04d797c61d54b06d52c680
Merge "PXE driver should not accept empty kernel UUID."