Update 17_rebuild_initramfs.sh

I updated the script to deal with changes introduced in Rhel7-Beta.
The rescue-images that are being created when installing a new kernel package should not be recreated by rear, as they
should contain all available modules. Only the hardware dependant images have to be recreated.
Additionally the initrd-plymouth.img from plymouth for graphical booting should not be touched.
Also two unneccessary lines were deleted.