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.

Enh: Daemon: set modulesctx.modules_dir when modules_manager is loaded.

Clean: ModulesContext.get_module() and ModulesManager.load().
Factorization should be easily do-able.

Conflicts:
	shinken/modulesmanager.py
As simple so.
Merge pull request #1348 from gst/enh_import_modules

Enh : modules management better load/import module functions.