Manage service and make device_driver configurable

This change does the following to lbaas agent:

- makes device_driver configurable, and changes default (current
  default is invalid)
- adds neutron_lb_agent_service attributes and manages said service

Change-Id: If6158abd01d1bbae849558d67c602073b54cdd27
Merge "Manage service and make device_driver configurable"