Images using draycut should have the option to pass drivers to include in build

[Dave]

I found that when creating images for use with ironic, I found that draycut.conf was creating images that were created only for the hardware in which it runs on (For efficiency).

When creating deploy images this is less than ideal. DIB should include the ability to define additional driver modules within an environment var.

In dracut.conf

hostonly="no" (If yes it is specific for the hardware)
add_drivers+="<passed in variable>"