lxc-ubuntu: use release-updates and release-security

lxc-ubuntu: use release-updates and release-security

Particularly for LTS releases, which many people will want to use in
their containers, it is not wise to not use -security and -updates.
Furthermore the fix allowing ssh to allow the container to shut down
is in lucid-updates only.

With this patch, after debootstrapping a container, we add -updates
and -security to sources.list and do an apt-get upgrade under chroot.
Unfortunately we need to do this because debootstrap doesn't know how
to.

Signed-off-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>