lxc-ubuntu: Guess a list of langpacks to install

lxc-ubuntu: Guess a list of langpacks to install

In addition to creating the current locale in the container, also
try to scan the host and extra the list of langpacks installed there,
then pass that list to debootstrap as additional packages to install.

On distros that don't have dpkg, only language-pack-en will be installed.
The code will always ensure that language-pack-en is ALWAYS installed in the
target, similar to what Ubuntu does with its various media.

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