Improve naming convention in ci configuration module

lib/ci/gitlab_ci_yaml_processor.rb

lib/gitlab/ci/config.rb

lib/gitlab/ci/config/entry/artifacts.rb

lib/gitlab/ci/config/entry/attributable.rb

lib/gitlab/ci/config/entry/boolean.rb

lib/gitlab/ci/config/entry/cache.rb

lib/gitlab/ci/config/entry/commands.rb

lib/gitlab/ci/config/entry/configurable.rb

lib/gitlab/ci/config/entry/environment.rb

lib/gitlab/ci/config/entry/factory.rb

lib/gitlab/ci/config/entry/global.rb

lib/gitlab/ci/config/entry/hidden.rb

lib/gitlab/ci/config/entry/image.rb

lib/gitlab/ci/config/entry/job.rb

lib/gitlab/ci/config/entry/jobs.rb

lib/gitlab/ci/config/entry/key.rb

lib/gitlab/ci/config/entry/legacy_validation_helpers.rb

lib/gitlab/ci/config/entry/node.rb

lib/gitlab/ci/config/entry/paths.rb

lib/gitlab/ci/config/entry/script.rb