Have waitforcluster.sh work as a strict entrypoint

[pjenvey]

waitforcluster.sh could work more like a typical docker entrypoint script: making it responsible for executing the container cmd which would be passed as its last argument.

The main benefit vs what's done now (integrating the container cmd into the entrypoint along w/ waitforcluster and '&&') is it could support using the default container cmd.

E.g.:

http://stackoverflow.com/questions/37194733/how-can-i-make-my-docker-compose-wait-for-it-script-invoke-the-original-contai

Not terribly important right now -- it just means we require steps always specify the command being ran