Allow for changing the default database name, username and password from
`drupal` to `app` by enabling the
`experimental.useNewDatabaseCredentials` feature flag in a project's
`build.yaml` file.
Refs: OD-27
Allow for overriding the project root from `/app` to something else,
such as `/var/www/html`.
The default is set within a new `build.defaults.yaml` file which the
project specific file is merged into.
Fixes#27