templates: Refuse to run unprivileged

Only the download and ubuntu-cloud templates work with unprivileged
containers, for all others, detect --mapped-uid and error out as early
as possible, recommending the use of the download template.

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