UI : The admin is able to add / remove buckets for a user

[Cedric-Magnan]

If a bucket is added / removed for a user, then, the credentials used for this bucket are removed from the project if no other bucket configured for the user comes from this project.

An even better solution would be to let the users update their buckets, but only if they have the permissions to do so. If so, the user needs to allow the service to create / remove credentials on his behalf though an OAuth2 client for each cloud provider...