Send required_nodes field to the iallocator scripts

This patch adds the 'required_nodes' field in the request dict for the
iallocator.

This means that the handmade-checks in the create instance can be
simplified, and that the dumb allocator can be made simple. Therefore
the patch also modifies it.

The patch also sends the disk_space_total to the script in realocate
mode and a small fix for showing errors (include stderr too).

Reviewed-by: ultrotter