init: Don't put cloud-init twice on cmdline

The BOOT_ARG computation was buggy since the value for the GRUB_1
configuration was owning both definition (with and without the
semi-colon backslashed).

The grub configuration looked-like :
 kernel /vmlinuz-2.6.32-431.11.2.el6.x86_64 nomodeset rw ds=nocloud-net\\;s=http://192.168.1.1/ ds=nocloud-net;s=http://192.168.1.1/

This is not a bug since the right one is the latest but that's better
having it right the first time.