Move CI stages configuration to new CI config

lib/ci/gitlab_ci_yaml_processor.rb

lib/gitlab/ci/config.rb

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

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

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

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

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

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

spec/lib/ci/gitlab_ci_yaml_processor_spec.rb

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

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

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