Centralize/include terraform module pipeline configs

I've created a new project https://ops.gitlab.net/gitlab-com/gl-infra/pipeline-templates with the initial intent to centralize common elements for pipelines/jobs running under https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules, though the same principle could easily be extended to other aspects of the code/infrastructure we manage.

1. [x] Create base `.gitlab-ci.yml` template for terraform module pipelines
1. [x] Validate template CI configuration in a terraform module pipeline
1. [x] Document pattern in `https://ops.gitlab.net/gitlab-com/gl-infra/pipeline-templates/blob/master/README.md`
1. [x] Propagate changes to remaining modules

## Ops MRs

### Getting the pipeline template working

- [pipeline-templatesdone!2](https://ops.gitlab.net/gitlab-com/gl-infra/pipeline-templates/merge_requests/2)
- [pipeline-templatesdone!3](https://ops.gitlab.net/gitlab-com/gl-infra/pipeline-templates/merge_requests/3)
- [pipeline-templatesdone!4](https://ops.gitlab.net/gitlab-com/gl-infra/pipeline-templates/merge_requests/4)
- [pipeline-templatesdone!5](https://ops.gitlab.net/gitlab-com/gl-infra/pipeline-templates/merge_requests/5)
- [pipeline-templatesdone!6](https://ops.gitlab.net/gitlab-com/gl-infra/pipeline-templates/merge_requests/6)

### Rolling template usage out to terraform-modules projects

- [x] [bootstrap!7]
- [x] [cloud-nat!8]
- [x] [cloud-sql!10]
- [x] [database-backup-bucket!8]
- [x] [generic-stor!15]
- [x] [generic-stor-redis!15]
- [x] [generic-stor-with-group!12]
- [x] [generic-sv-sidekiq!15]
- [x] [generic-sv-with-group!10]
- [x] [gke!23]
- [x] [https-lb!4]
- [x] [monitoring-lb!3]
- [x] [monitoring-with-count!12]
- [x] [project!20]
- [x] [pubsubbeat!11]
- [x] [stackdriver!3]
- [x] [static-objects-cache!2]
- [x] [storage-buckets!15]
- [x] [tcp-lb!3]
- [x] [vpc!8]
- [x] [web-iap!4]

[bootstrap!7]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/bootstrap/merge_requests/7
[cloud-nat!8]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/cloud-nat/merge_requests/8
[cloud-sql]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/cloud-sql/merge_requests/10
[cloud-sql!10]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/cloud-sql/merge_requests/10
[database-backup-bucket!8]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/database-backup-bucket/merge_requests/8
[generic-stor!15]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/generic-stor/merge_requests/15
[generic-stor-redis!15]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/generic-stor-redis/merge_requests/15
[generic-stor-with-group!12]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/generic-stor-with-group/merge_requests/12
[generic-sv-sidekiq!15]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/generic-sv-sidekiq/merge_requests/15
[generic-sv-with-group!10]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/generic-sv-with-group/merge_requests/10
[gke!23]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/gke/merge_requests/23
[https-lb!4]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/https-lb/merge_requests/4
[monitoring-lb!3]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/monitoring-lb/merge_requests/3
[monitoring-with-count!12]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/monitoring-with-count/merge_requests/12
[project!20]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/project/merge_requests/20
[pubsubbeat!11]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/pubsubbeat/merge_requests/11
[stackdriver!3]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/stackdriver/merge_requests/3
[static-objects-cache!2]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/static-objects-cache/merge_requests/2
[storage-buckets!15]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/storage-buckets/merge_requests/15
[tcp-lb!3]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/tcp-lb/merge_requests/3
[vpc!8]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/vpc/merge_requests/8
[web-iap!4]: https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/web-iap/merge_requests/4

Story points: 2.0