Winery settings refactorings

The final goal of this series of refactorings is to make the shards pool configurable (between a RBD-based pool and a file-based pool).

As all the winery components had been written to take a bunch of indiscriminate kwargs and picking through them, adding more settings would be a pain, so I went ahead and moved settings to a clearer (I hope) hierarchy, with the aim of reducing the number of kwargs passed around.

Eventually this should allow performing the "file-based pool" refactoring.

This only makes sense in a commit-by-commit review, I think...