Migration handlers: drastic strategy

[vincent-zurczak]

We should add a new aspect to Roboconf: continuous deployments. This implies we need to be able to migrate an application. And there are several strategies. We should start with the drastic solution.

It consists in undeploying the previous application and then deploying the new one. It also means there will be a service interruption. Although it is not ideal, it is the first step to implement. It will also help to break the big Manager class into several ones.

Indeed, loading an application and restoring it can be handled in a specific migration handler. Said differently, deploying and restoring applications are special cases of migration. This refactoring work will also prepare the field for new migration strategies.