lxc-ubuntu: stop early if a bad user is specified in -b option
lxc-ubuntu: stop early if a bad user is specified in -b option

Otherwise we end up with a bad container fstab and a container
that won't boot.  See
https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/879052

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@canonical.com>
Signed-off-by: Daniel Lezcano <3211518dd4d87cb3f7ee12cb52e14b7fb2bc0446@fr.ibm.com>

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