This isn't a feature we've been using as far as I know and if someone
needs a custom partition layout it's probably better to just add it to
the json file. Removing this avoids some complexity.
Move from optparse to argparse. Move layout file and layout type to
global options with reasonable default values so every command doesn't
need to them. Adjust calling scripts to match.
For now layout type is being passed via an environment variable
DISK_LAYOUT_TYPE but this is a temporary situation.
