Create a common place for Load Tests configuration

The Apache Beam community maintains different versions of each Load Test. For example, right now, there are two versions of all Python Load Tests: the first one runs on Dataflow runner, and the second one runs on Flink. With the lack of a common place where configuration for the tests can be stored, the configuration is duplicated many times with minimal differences.

The goal is to create a common place for the configuration, so that it could be passed to different files with tests (.test-infra/jenkins/*.groovy) and filtered according to needs.