New extension for version 2 of LBaaS API

Also added some constants for supported protocols, algorithms,
health monitor types, and session persistence types.

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

Change-Id: I43270fdf9cd52a57274575ecea502f660ff0d5a9
Co-authored-by: Brandon Logan <brandon.logan@rackspace.com>
Co-authored-by: Vijayendra Bhamidipati <vbhamidipati@paypal.com>