Currently diskimage-builder supports two ways to specify the image
size. One is defining a fixed image size using DIB_IMAGE_SIZE, the
other one is auto-detection while adding a security margin of 60% as
free space. This means when building larger images (e.g. >100GB) with
unknown size upfront we end up with much wasted space, IO and network
traffic when uploading the images to several cloud providers. This can
be optimized by adding a third way by defining DIB_IMAGE_EXTRA_SIZE to
specify the free space in GB. This makes it possible to easily build
images of varying sizes while still minimizing the overhead by keeping
the free space constant to e.g. 1GB.
Change-Id: I114c739d11d0cfe3b8d8abc6df5ff989edfb67f2
