Store when and yaml variables in builds table

## What does this MR do?
Stores `when` and `yaml_variables` in `ci_builds` table.

## Why was this MR needed?
This is done in order to simplify the code responsible for creating a pipeline builds.

## What are the relevant issue numbers?
This made as a pre-step for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5295.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5296