add "rd.auto" parameter and switch off automatic assembly

add "rd.auto" parameter and switch off automatic assembly

No automatic assembly is done anymore by default. You will have to
specify exactly what devices to assemble
("rd.md.uuid=" "rd.luks.uuid" ...)
or use "rd.auto=1" or "rd.auto" on the kernel command line.

For big servers with thousands of disks we don't want to assemble
everything by default (error prone, slow).