Delete baremetal image files after deployment.

Unlike libvirt where the image files are needed for running VM's, Nova
baremetal copies them to the remote machine, after which they are no
longer needed. Keeping them sharply reduces the scalability of Nova
baremetal by limiting a single nova-compute to deploy instances whose
disk size sums to it's local disk - but really only needs to be a
concurrency limitation + the kernel/ramdisk files, which are stored
separately.

Change-Id: I7f4791cbd3cff3a0edaccd308c1c47179a4fcf78
Co-Authored-By: Steve Kowalik <steven@wedontsleep.org>
Merge "Delete baremetal image files after deployment."