Add new 'openssh-server' element to ensure that openssh server
is installed and enabled during boot. This is mostly useful for
*-minimal images which do not come with openssh installed and/or
enabled in order to keep a small dependency footprint.
Change-Id: Ide15ee04f5de123dbc8ce4bb56d638d8a167c341
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