Minor reorganization here so the --dry-run option is listed at the end,
and we document that the --timeout option takes an argument. I had also
missed showing the `[-d]` option on the usage message previously.
I also slightly re-organized how the default was set, so it will always
rely on the status of the $timeout variable and also the final variable
name matches the name of the Packer option.
This does not yet call the actual `packer build` command, but it does
grab the metadata correctly. I'll have to change up the option flag
parsing to include the provider type, then it can just call the complete
command rather than passing options through.
