Cleanup: remove deprecated fields the proper way

Also move converters to their own file to make it easy to move them into v1beta1 later.
Here I changed it to ```b := fmt.Sprintf("%s:%s%s", vol.GetPath(), mount.MountPath, map[bool]string{false: "", true: ":ro"}[mount.ReadOnly])``` :) :)

...OK, I'll change it back.