Increase the size heuristic for images

Occasionally cloud-init fails to resize the disk on first boot, this is
occuring because the filesystem has only 10% free. Nudging the disk size
up a bit more should give us a little more headroom so this is less
likely to occur.

I havn't been able to find out what value ensures we wont hit this
problem but I can say for sure this fixes all casses of the problem I
have seen.

Change-Id: Ib23f3a654151338ad91839e49b323b65b4054245
Merge "Increase the size heuristic for images"