allowing configuration of soft/hard limit for nova on computes

I thin it would be a nice addition to allow a user to configure how much of the current compute storage they would like nova to actually use.
In my setup installed with packstack, nova uses the entire disk mounted by root which means that we can crash the server if an instance starts writing and filling the disk.

in my current installation (deployed by packstack) nova uses the entire partition mounted on root:

[root@orange-vdsf nova(keystone_admin)]# fdisk -l

Disk /dev/sda: 73.3 GB, 73284976640 bytes, 143134720 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x0007ab59

   Device Boot Start End Blocks Id System
/dev/sda1 2048 4095 1024 83 Linux
/dev/sda2 * 4096 1028095 512000 83 Linux
/dev/sda3 1028096 15341567 7156736 82 Linux swap / Solaris
/dev/sda4 15341568 143134719 63896576 5 Extended
/dev/sda5 15343616 143134719 63895552 83 Linux
[root@orange-vdsf nova(keystone_admin)]# df .
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda5 62760684 17879540 41669984 31% /
[root@orange-vdsf nova(keystone_admin)]#