This updates the various checks to match the grid below:

== lxc-ubuntu support per architecture ==
amd64: amd64, i386, armel, armhf, powerpc
i386: i386, armel, armhf, powerpc
armel: armel, armhf
armhf: armhf, armel
powerpc: powerpc

== lxc-ubuntu-cloud support per architecture ==
amd64: amd64, i386
i386: i386
armel: armel, armhf
armhf: armhf, armel

Note that most of the foreign architectures on x86 are supported
through the use of qemu-user-static. This one however isn't yet
support for cloud images (I'll send a patch for 1.0).

Also, qemu-user-static is technically able to emulate amd64 on i386
but qemu-debootstrap doesn't appear to know that and fails quite miserably.

We may also want to add a test for amd64 kernel but i386 userspace, which
is a valid combination that allows running an amd64 container on an i386
host without requiring emulation, but that's for another patch.
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>