Use the new argo_nodeos_config --exec feature in development.
Allow us to delegate fork+attach+exec to argo_nodeos_config, and
simplifying the create command as a result.

We use tornado.process to wrap this command, as we want to able to
stream stdout/stderr in the future.

This patch also misuse, the 'pid' field of the container namedtuple to
save the tornado.process.Subprocess object itself, so some functions
need to be adapted.