templates: Disable the os-prober by default

templates: Disable the os-prober by default

The os-prober is enabled by default what may lead to potentially
dangerous use cases and borderline opening attack vectors. This
patch disables the os-prober, adds warning messages and updates
GRUB_DISABLE_OS_PROBER configuration option documentation. This
way we make it clear that the os-prober usage is not recommended.

Simplistic nature of this change allows downstream vendors, who
really want os-prober to be enabled out of the box in their
relevant products, easily revert to it's old behavior.

Reported-by: NyankoSec (<nyanko@10x.moe>, https://twitter.com/NyankoSec),
             working with SSD Secure Disclosure
Signed-off-by: Alex Burmashev <alexander.burmashev@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>