Feature/database separation

Completes #146.

To spin up a DB server, set up your `.chef/knife.rb` then

```sh
bundle exec knife ec2 server create -G myusa-staging-db --identity-file $AWS_KEY_PAIR_PATH --ssh-user ubuntu --node-name 'MyUSA staging app' -r 'role[app]' -S $AWS_KEY_PAIR_NAME
```

Then grab the IP address/hostname and set up an app server accordingly:

```sh
bundle exec knife ec2 server create -G myusa-staging-web --identity-file $AWS_KEY_PAIR_PATH --ssh-user ubuntu --node-name 'MyUSA staging app' -r 'role[app]' -S $AWS_KEY_PAIR_NAME -j '{"myusa":{"database":{"host":"[DATABASE IP ADDRESS GOES HERE"}}}'
```



Weird, the diff isn't showing up properly. You can see it here: https://github.com/18F/myusa/commit/b652986fefa6f0899e69f57a72cf9514b9bd0619

I just added placeholders for the AWS SES keys.