Added --chroot-sessions option to kernel line

By default, Ubuntu disables chroot support, which means upstart service
handling does not differentiate between services started by the host,
and services started in the chroot. This causes problems, like installing
ssh in the chroot fails when the host system doesn't already have ssh
installed, because the ssh postinst scripts try to start the chroot
service, which upstart doesn't know about (because it wasn't installed
on the host). --chroot-sessions tells upstart to keep chroot services
separate from ones started on the host