Manage config for saving riemann dashboard config to s3.

This is pretty much working, but might need some cleanup to get up to snuff (I'm still relatively new to puppet). One known hole is that the default value for for `use_s3` doesn't work. I don't know how to fix it, but I figure you might (or perhaps my fine fellow udacian, @ods94065, knows how?).

Happy to work out any concerns you have to get this merged, just let me know what needs doing. Would be nice to get #14 in shortly thereafter, too, so we can switch back to using the version from the forge.
I think with the use of undef suggested elsewhere this isn't required.