Refactor CI so that the YAML processor is decoupled from Ci::Pipeline

Follow up from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4466: right now Ci::Pipeline defines #config_processor which will parse the .gitlab-ci.yml and memoize the result. This code should be decoupled from the model so it's easier to alter its behaviour (e.g. whitelisting the jobs to build). Probably the best place to call this from would be Ci::CreatePipelineService.