ubuntu: Tweak architecture support

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>
Set all mounts to MS_SLAVE when starting a container without a rootfs

If the filesystem mounts on the host have the MS_SHARED or MS_SLAVE
flag set, and a container without a rootfs is started, then any new
mounts created inside the container are currently propagated into
the host. In addition to mounts placed in the configuration file of
the container or performed manually after startup, the automatic
mounting of /proc by lxc-execute will propagate back into the host,
effectively crippling the entire system. This can be prevented by
setting the MS_SLAVE flag on all mounts (inside the container's own
mount namespace) during startup if a rootfs is not configured.

Signed-off-by: David Ward <david.ward@ll.mit.edu>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>