keygen: Reflect change in knet

keygen: Reflect change in knet

Knet commit 1cb36f0cffd4559971826ca4774a88c5b05882fb reduced minimal
key length to 1024-bit. Keygen should keep compatibility with already
released 3.0.[0-1] so default key length should be 2048 bits. It's
possible to use -s argument to generate shorter key - keygen respects
minimum/maximum as defined by knet.

Also fix man page to reflect this change.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>