Remove useless properties to use the configuration property, ensure that
expected configuration is well provided and add specs
Add missing methods in product repository interface
Move DISPLAYED_ATTRIBUTES_PATH from product datasource to context
configurator
Move USEABLE_ATTRIBUTES_PATH from ProductDatasource to ContextConfigurator
Fix CS
Merge pull request #961 from akeneo/spec/product_datasource

[WIP] Spec/product datasource