Remove usage of Glance from PXE clean_up()

Save uuids of instance kernel and ramdisk from Glance into node driver_info
('pxe_kernel' and 'pxe_ramdisk'). Stored values used in PXE clean_up()
method.

Closes-Bug: #1300602
Change-Id: I59b3dbd4f66c59060e5b229eae00b80fdc4702e5
Merge "Remove usage of Glance from PXE clean_up()"