ubuntu: Fix regression in post-process

THe recent reorg of lxc-ubuntu introduced some package installation in
post-process but without first disabling service startup.

As a result, if the cache is a bit out of date and a ssh update is
available, post-process will apply that update (as it does apt-get
install ssh vim) which in turn will attemp to start sshd. This will
either lead to ssh on the host being restarted or if there's no sshd on
the host, will fail the container creation as the postinst will get an
error from upstart.

The fix is very simply to add the same policy-rc.d trick when running
post-process.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>