Make reference lbaas implementation as a pluggable driver

implements blueprint multi-vendor-support-for-lbaas-step1

This patch implements the following changes:
 * merge lbaas_plugin.py and plugin.py into 'plugin.py'
   After that the default 'reference' implementation is available again.
 * move all code related to reference implementation from plugin.py to
   drivers/haproxy/plugin_driver.py
 * Inherit HaproxyOnHostPluginDriver from abstract driver and implement
   its interface.
 * modify tests accordingly

Change-Id: Ib4bfe286826acdedeadbeeff4713448c073378d2