Important services such as L3 routing and lbaas (load balancing) are currently supported in Quantum's API as extensions. The goal is to move to move support to the core API for simplicity and ease of maintenance. https://blueprints.launchpad.net/neutron/+spec/api-core-for-services
The only change that merged [1] in support of the blueprint allows core plugins to expose service implementations that previously had to be defined separately. This precludes a general testing strategy, and evaluation of plugin-specific functionality will automatically cover the change in question. 1: https://review.openstack.org/#/c/28196/6