User is never passed into the node-execute step command

[ryancurrah]

In the node-execute step the user parameter is passed into the step but is never passed to docker exec command.

docker/docker-container/contents/node-execute

Line 57 in b7b256f

shlex.split('docker exec %s %s' % (args.container, args.command)),

Just a note by fixing this issue it may break some users existing steps as the steps will now run as root, which may not be expected.