Split build playbook into smaller steps

[cameel]

Currently we have one big concent-builder/build-test-and-push.yml playbook which builds containers, builds Kubernetes configs for the cluster, pushes images to our Docker registry and runs unit tests. This was convenient when we had to deploy manually every time, but now that we have CI (#284), it would actually be more convenient to be able to run each step separately. Split this playbook into:

• build-containers.yml, parameters: use_docker_cache (this parameter will be added in Allow enabling/disabling docker caching in the build playbook #293)
• build-cluster-config.yml, parameters: cluster
• run-django-tests.yml, no parameters
• publish-docker-images.yml, no parameters

Then make CI run each of them as a separate step.

Update build instructions in the README.

[PaweuB]

Pivotal:
https://www.pivotaltracker.com/story/show/163085230