New rootfs built from clout apt packages

- Remove debootstrap entirely, use multistrap instead as it supports
packages from more than one source
- All multistrapping packages come from cloud APT, and sources.list in
the final image point to cloud.
- The package install/update step at the end is slightly different, as
keys need to be installed first, due to all packages from cloud being

Change-Id: I18188ff557f1043e8786dd4bb9a9439dc8d42472
13 files changed