Move global summary log formatting into the custom formatter.

This makes our custom json log formatter take over some of the responsibilities of the summary log line, such as including the hostname in each record.  It also makes it cope better with string message log items, and disables logging of the summary info by default.

The idea is that the default logging setup uses pretty-printed console output and does not include the verbose summary output, so it's useful for running a devel or debug server.  In production, we switch on the summary log lines and send it all to stdout as heka-ready json like this:

```
{
  "logging": {
    "handlers": {
      "console": {
        "class": "intel/handlers/console",
        "formatter": "json"
      }
    },
    "loggers": {
      "bid.summary": {
        "propagate": true
      }
    }
  }
}
```

This makes the production setup look more similar to fxa-auth-server, and will make for a simpler deployment with circus as discussed in https://bugzilla.mozilla.org/show_bug.cgi?id=996763#c11.

@seanmonstar r?