Explain difference between 'DIB_OFFLINE' and
'DIB_DEBIAN_USE_DEBOOTSTRAP_CACHE'
Those variables are not redundant,they hava different effective ranges.
However,some people may be confused about this and reported a bug.
So,this difference should be writen in the README file.
Closes-Bug: #1506275
Change-Id: Ie5316de41d129bf98781708954f09ef0b2592b53
debootstrap is not debian or ubuntu specific. We can make a debootstrap
element that knows how to do all of the things, and then a
debian-minimal and ubuntu-minimal image that use it. Finally, make
the debian element simply be a collection of the extra things we do to
make it look like a cloud-init based cloud image.
Change-Id: Iaf46c8e61bf1cac9a096cbfd75d6d6a9111b701e
