Flesh out docs for smartstack.yaml

[EvanKrall]

This adds documentation for all of the keys listed in the key_whitelist in marathon_tools.py.

[mrtyler]

1. Is that markdown formatting correct?
2. I assume you copied this from some other (internal) doc? If so, do you have a plan to kill the internal-only one or otherwise prevent the copies from drifting from one another?

[EvanKrall]

1. This is ReStructured Text, not Markdown. Code blocks are started by ending the previous line with ::
2. This is new content, not copied from internally. Our internal docs are more substantial, but somewhat tied to our configuration. I'll try to pull that content into this.

[solarkennedy]

Looks fine to me. Could be more RSTy but a well commented config file is just fine by me.

[EvanKrall]

I've copied a bunch of info from our internal smartstack docs and RSTified it.