fix #4236: split disk space check into soft and hard limits

fix #4236: split disk space check into soft and hard limits

Make the 8 GB disk check a soft limit, as most products are still
installable with less space, even though it may need some special
options in the disk selection. Still notify the user with a prompt to
avoid an expected possible error seem like it was unexpected.

Keep 2 GB as hard limit for now, makes no sense to install on such
small devices, only pain.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>