It turns out we still have some backend code relies on the flask app,
so revert the change to init_config, and make minor change to create
config object for frontend and backend with the same function.
1. init_web_config: create Config object for frontend, load
configuration from `web_config.py`.
2. init_backend_config: create Config for backend, load configuration
from `backend_config.py`.
And two new classes inherit from `Config` in config.py:
1. WebConfig: representing the orchestrator frontend web configuration
2. BackendConfig: representing the orchestrator backend workers
configuration
Before calling init_{web,backend}_config, check sys.argv, if
"fedmsg-hub*", "celery" or "build_module_locally" is present, it's
running as backend.
