download: Initial template

download: Initial template

This adds a new template called "download". It's a fairly simple
template with a minimal set of dependency which will grab any pre-built
image available on https://images.linuxcontainers.org
Note that the serverside is still work in progress (missing SSL support).

Access is done over https by default with a warning being emitted if
fallback to http was required (may be needed for testing, when behind
proxy and with private servers). All index files and tarballs are
gpg-signed with the default pubkeyid contained in the template itself.

The main benefit of this template is to be entirely
distribution-agnostic, any template that can be integrated with the
server build infrastructure will then work on any LXC machine when using
the download template. This template is also compatible with user
namespaces and will hopefully help widden the number of distros that may
work in unprivileged LXC.

This commit also bundles a small change to the template configs to have
the ubuntu template (used by the download template) to work with
unprivileged LXC.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>