lxc-ubuntu: Support for building a container of a foreign architecture

lxc-ubuntu: Support for building a container of a foreign architecture

Support building a container of a foreign architecture if
qemu-user-static is installed.  This is done by installing some packages
of the host architecture in the container using multi-arch.

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