Enable parallel builds using diskimage-builder

Background:
Currently if you run parallel builds for diskimage-builder on a system the builds fail, because the diskimage-builder uses the common “~/.cache/image-create” directory to cache OS image files, git repositories, Python or Ruby packages etc. to create the images.

Proposal:
If we want to enable parallel builds, then we need to identify and tie the builds to the correct “~/.cache/image-create” folder. To do this we can use mktemp and let diskimage-builder use this directory as the cache directory instead of “DIB_IMAGE_CACHE” (~/.cache/image-create).

    export CACHE_DIR=$(mktemp -d ~/.cache/image-create.XXXXXXXX)

This will create the "~/.cache/image-create.XXXXXXXX" directory and let the diskimage-builder use it for its respective builds.

Also, to enable diskimage-builder to build with the correct "~/.cache/image-create.XXXXXXXX" directory for “--offline” mode; the script should force the user to specify the respective “image-create.XXXXXXXX” folder to refer to for the build.