Feature: Add Load Balancer API

[phip123]

Currently the system does not have any representation of a load balancer, which should be integrated to build optimization programs that can implement strategies for all main components: load balancer, scaler and scheduler.

The testbed load balancer implementation (go-load-balancer) gets weight updates via etcd.

This makes it plausible to create an API that just exposes a method to update these weights.
The implementations are responsible to update the underlying load balancer instances but they are required to be weighted round based.