modules management: fix, enh&clean of load/import module functions.

Fix: now possible to import as true packages the shinken modules. 
Even if they do relative import. So we don't over populate sys.path with every module path.

Enh: Daemon: set modulesctx.modules_dir when modules_manager is loaded.

Clean: ModulesContext.get_module() and ModulesManager.load().
Factorization should be easily do-able.

Well, I don't agree because it's perfect for such use case :p