Ensure machine doc is inserted last after any related docs

Maybe fixes: https://bugs.launchpad.net/juju-core/+bug/1354027

With debug turned on, it could be seen that the provisioner task was being notified of containers added to a host, which the provisioner then started to process. However, at this time, the machine records were not necessarily all written to the database yet. It turned out that for one container, its status document was not yet written, so the provisioner saw an error and ignored the container forever. A couple of seconds after this happened, the status record was written.

The txn ops to add a machine and its associated docs are re-ordered such that the main machine doc is created last.