PXE driver should rmtree directories it created

Baremetal PXE driver was failing to delete the per-instance tftpboot and
image directories which it created when the instance was deleted. This
happened partly because of dangling files within the directory, and
partly because 'unlink' does not remove directories and the error was
squelched.

Now, when destroy()ing an instance, PXE driver will call shutil.rmtree()
on the per-instance directories which it created.

Fixes bug 1101048.

Change-Id: I682d0264288add488ea23e5d5200357b7be52dd9
Merge "PXE driver should rmtree directories it created"