More traits for pipelines factory

We now have a number of different create_pipeline defined in the tests, which if possible, we should really use traits to capture common patterns. A common pattern is:

create(:ci_pipeline,
       project: project,
       ref: project.default_branch,
       sha: project.commit.sha)

Could traits have arguments? So that we could have ref as the argument here.

Related discussion: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5862#note_14043923

/cc @grzesiek