Improve sandbox URI configuration

[fbiville]

While sandbox.repositories is technically externalizable, defining the list in YAML is easier.

Because of that, the configuration of sandboxes to watch is versioned with the application.

It would be best to configure this list as a CI parameter (envvar), but we need to find a way to define the setting that is not cumbersome.

[fbiville]

This could currently be done via https://github.com/spring-projects/spring-boot/wiki/Relaxed-Binding-2.0#lists-1.