Improve code, remove unused validator, improve names

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

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

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

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

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

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

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

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

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

spec/lib/gitlab/ci/config/node/artifacts_spec.rb

spec/lib/gitlab/ci/config/node/trigger_spec.rb