Fix the bootfile_name method call in baremetal

nova baremetal was supplying a pxelinux config file disk path, but
PXE clients need the binary to bootstrap, which is neither the config
file, nor a filesystem path - it's a path within TFTP server.

As some environments/architectures may have different PXE binary,
this needs to be an option.

DocImpact

Change-Id: I393552ae2d45e1ff5bc306fc8717fa4265ac827f
Closes-Bug: #1220904
Merge "Fix the bootfile_name method call in baremetal"