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.

> Remember that most of the contributors are NOT dev :) They are admins that help to code with new features, and I won't allow too much magic that can hurt their contribution :)

That's why we are here to review every pull request : ensure code quality. IMO this is not magic, just using python syntax. This kind of use is fine as long as comment are clear. 
 
>Ps: I already let a dev make its magic in the project, and we work lot of hours to get back to a maintainable state. Never again :)

Then it was not a good one :) Travis and Jenkins are here to avoid such fail.