When a container is installed with 32bits binaries while we are
running on a 64bits host, inside the container we are seen as
64bits arch. That leads to some problems for the package updates
because the scripts will download 64bits packages instead of 32bits.

This patch defines a configuration variable to set the architecture
of the container.

lxc.arch = i686 | x86 | x86_64 | amd64
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>