Basic functionality of device manager

The new daemon connects to rabbitmq and listens to messages from API server and takes action as required.
DeviceManager class in device_manager.py provides client connections to rabbitmq and cassandra.
db.py has database classes to maintain local copies of data. It also maintains links across objects.
dependency_tracker.py provides a way to track dependencies among different objects.
physical_router_config.py provides interface to physical routers. At present, it is based on netconf, but in future, it could use any transport protocol. The rest of the code does not make any assumptions about about netconf.

Test cases for device manager are being added as a separate review request.

Also remvoved all netconf code from schema transformer.

Change-Id: Ied0de2226f9562b84a9ba6cd77bb930c896ffb20
Merge "Basic functionality of device manager"