Use new images when spawning BM instances.

With nova-BM the TFTP files (kernel and ramdisk) are cached between
instances but disks never are (today). We were treating the disk image
the same way but this is incorrect because its filename is 'disk'
rather than '$uuid' and as such it has to be cleaned out whenever a
deploy request is being processed.

Change-Id: I15c8fc25a4c064341b9f1ed51e6df8d1494fbdff
Co-Authored-By: Robert Collins <rbtcollins@hp.com>
Merge "Use new images when spawning BM instances."