[wip] Monitoring

I'm adding Cloud Monitoring checks to the site as per #141, but first I'll need to write the Ansible modules to create entities, checks, alerts and so on.

/cc @rgbkrk 
@rgbkrk, @ycombinator: As I'm writing this now, I'm setting up monitoring on each webserver individually, not the load balancer. That's probably not what we want, is it? With #317, would that even work?

I'm thinking about changing this from a role applied to each webserver to a `connection: local` play that's included into `site.yml` after the rest of the system is provisioned that monitors in front of the load balancer instead of behind it.

What do you guys think?