Plugin/DB additions for version 2 of LBaaS API

Added alembic migrations, models, db methods and plugin methods.
Added DEFERRED status for entities not linked to a load balancer.
Added to_dict method in BaseNeutron model.
Sql Alchemy models in its own module.
Plugin database methods are accessed by composition through the plugin.
Added data models to convert sql alchemy models before passing to driver.

Partially-implements: blueprint lbaas-api-and-objmodel-improvement

Change-Id: Iaec2c2315ae54418b341eb31e6b3ff53b02ca9d7
Co-authored-by: Brandon Logan <brandon.logan@rackspace.com>
Co-authored-by: Vijayendra Bhamidipati <vbhamidipati@paypal.com>
Co-authored-by: Craig Tracey <Craig.Tracey@gmail.com>
Co-authored-by: Doug Wiegley <dougw@a10networks.com>