Replace the dummy broker with the async bookstore one

[danail-branekov]

Background

According to the OSBAPI docs, if one wants to implement a broker, they should start with the spring boot template project. Its reference implementation is the bookstore broker. That broker is however synchronous and we want to test against a broker that supports the more complex async flow

We plan to contribute the async bit back to the bookstore broker. Even if the PR is not accepted, we could get rid of our dummy broker and use the async bookstore broker fork instead

Such an approach would make us more confident that we are testing against a real-life broker and that our osbapi client implementation (and our OSBAPI understanding) is correct.

Actions to take

• Setup CI to build the async bookstore broker as a docker image and push it to e.g. dockerhub
• Configure tests to start the bookstore broker and run against it
• Remove our dummy broker