Store retried in database for CI builds

app/models/ci/build.rb

app/models/commit_status.rb

app/services/ci/process_pipeline_service.rb

app/services/ci/retry_build_service.rb

app/services/ci/retry_pipeline_service.rb

...gs/unreleased/store-retried-in-database-for-ci-builds.yml

db/migrate/20170503004426_add_retried_to_ci_build.rb

db/post_migrate/20170503004427_upate_retried_for_ci_build.rb

db/schema.rb

spec/lib/gitlab/import_export/safe_model_attributes.yml

spec/models/ci/pipeline_spec.rb

spec/models/ci/stage_spec.rb

spec/models/commit_status_spec.rb

spec/requests/api/commit_statuses_spec.rb

spec/services/ci/process_pipeline_service_spec.rb

spec/services/ci/retry_build_service_spec.rb

spec/services/ci/retry_pipeline_service_spec.rb

spec/views/projects/pipelines/_stage.html.haml_spec.rb