Create database repositories

Need to create database repositories to do the CRUD operations of entities. Neutron LBaaS had a giant monolithic modules and class that did them all. I'd rather have them separated out by entities. For Example:

octavia/db/repositories/loadbalancer.py

class LoadBalancerRepository
...

Or there could be one module with all the classes in it. For example:

octavia/db/repository.py

class LoadBalancerRepository
...
class ListenerRepository
...