lxc-ubuntu-cloud: Default to trying released channel

This adds a fallback to the daily channel if an image couldn't be found
in the released one.

Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
Add missing doxygen tag

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>