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.

Even if it's not so common it's still totally explicit... It's only a matter of a so small effort to know (and be used) to this "for else" syntax construct.
I don't see any Python feature/syntax that sucks here, it's possible that some syntax/feature could suck in certain occasions but not because they suck inevitably but because they could be badly used.
Also I don't see added complexity too but removed. At no cost but a comment which was already present.
We use plenty of Python "strange" or "magic" features : metaclass, inheritance, decorators, dynamic evaluation (eval), and so on... (well about everything that I personally like with Python and that I think make it the second best language in the world, the first having still to be invented). But you are stuck with this block of 5 lines (previously 8) because there is an else after a for loop block ?  I just don't get it :s