Restructured code based on the new design for cfg agent and helpers

Main changes:
Added a service_helpers directory and a device_drivers directory where
the respective service helper classes and device driver classes are kept.
Cfg agent acts a high level container for these. Agent loads the
service helpers. Only routing service helper is present currently.
DeviceDriverManager loads the CSR routing drivers. The drivers implement
the (routing) service interface present in services_api.py.  There is also
a dummy routing driver for testing without CSR1k.

Change-Id: Ie8baadd27325b9fd4048dac79513b7cde7180637