Push QA images to registry

This should address gitlab-qa#81 (comment 44232938). Also part of https://gitlab.com/gitlab-org/gitlab-ce/issues/39637.

1. Add Build::QA to handle QA-specific logic
2. Add a new Build::Image module that is extended in Build::GitlabImage and Build::QAImage.
3. Build::GitlabImage and Build::QAImage only define dockerhub_image_name and gitlab_registry_image_name so that all the get, tag, push logic is centralized in Build::Image
4. Renamed the GitLab QA images to gitlab-{ce,ee}-qa and removed the edition from the tag since it would be redundant.
5. Push the QA images after building it so that gitlab-qa (the gem) will use this image when running Component::Specs#perform in the qa:test task.
6. Improved the tests and coverage of all the classes under lib/build
7. Improved the tests and coverage of lib/docker_operations.rb
8. Improved the tests and coverage of lib/tasks/docker_tasks.rake and lib/tasks/qa.rake

Closes gitlab-qa#81 (closed) and gitlab-qa#106 (closed).

/cc @grzesiek