lxc-ubuntu: Use dpkg --add-architecture
lxc-ubuntu: Use dpkg --add-architecture

When a container has dpkg >= 1.16.2, use dpkg --add-architecture
for multi-arch configuration on foreign architecture containers.

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>
Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>
Signed-off-by: Daniel Lezcano <3211518dd4d87cb3f7ee12cb52e14b7fb2bc0446@fr.ibm.com>

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