Neutron/LBaaS/PluginDrivers

Driver framework for Loadbalancer Plugin

Rationale:
There may be a need to implement different architectures depending on the nature of the solutions. Some of possible options are the following:

1) Current reference implementation:
All balancers are HAProxy processes on the single host

2) Reference implementation improved by agent scheduling
balancers could be created on different hosts on which balancer agents are running. Agent (and therefore, host) is chosen by scheduler.

3) REST Proxy to external device management component
Plugin sends REST request to external component that chooses device and configures it

4) Device inventory/scheduling and multi-vendor agent
Plugin chooses device using device inventory and sends requests to agent, which applies configuration changes via device-drivers

To avoid making separate plugin for each of architectures we may introduce plugin-side drivers which implement interaction with other components of the service (agents, external systems or devices) in the way most suitable for the specific vendor solution.

Here are examples of possible workflows: 1) Current reference implementation