Allow spawning VMs in different regions

[Jacopo]

Right now all VMs are created in us-west-2 -- it would be nice to also offer a couple other regions as options.

It shouldn't be too hard, spawner.py is virtually ready.

On the OOO side, the AWS user / role will need to be expanded to allow some of the regions we're not using for the games.

We'd also record the user's preference in UserSettings: default_region is already there for this, and ideally geo-locate by default to suggest a good default.