Symfony 4 support

[stof]

AFAICT, the codebase should be compatible with Symfony 4 already. But the documentation is not (or the codebase if we don't want to change the doc at all).

The documentation is suggesting to use both the CloudManager and the clouds as public services. But services will be private by default in Symfony 4.
I suggest we keep them private, but document how to use them through service injection and autowiring (autowiring is not yet supported by default, but PR is coming)