Add test for autodiscovery DC filtering

This PR adds an integration test for autodiscovery DC filtering. I'd like to start using autodiscovery in production, but the implications of a regression would be a potentially crippling amount of cross-DC traffic, and huge latency on the client. I'd feel much better if it were covered.

I had to reshuffle the ccm commands a bit (it took a few attempts to figure out how to the multi-DC stuff working... turns out 'updateconf' blows away the DC properties).

This only tests DC filtering, as rack filtering would require more ccm voodoo than I felt like doing, and is likely more code than it's worth (I'm happy to add it though, if folks feel it would be helpful)