Consider supporting live ISO console customization

[bgilbert]

#977 introduces install --console and iso/pxe customize --dest-console to customize the bootloader and kernel console in the installed system. For live PXE boots we don't control the bootloader configuration or kernel arguments, but in the live ISO we do. Consider adding iso customize --live-console to configure the bootloader and kernel console in the live system. This would require supporting configuration syntax for every bootloader used in the ISO image, currently ISOLINUX and GRUB for x86_64.

Additionally, we could add iso customize --console as sugar for --dest-console X --live-console X. Note that --dest-console invokes the installer as a side effect, so --console isn't a complete replacement for --live-console.

I don't think there's a broad use case for this, but it might be useful in unusual situations.