Revert "hack/yaml-lint: No-op this script"

This reverts commit 8ff1cee
(2018-09-28, openshift#370).

We moved from Glide to dep in 1f45543 (vendor: switch from glide to
dep, 2018-09-28, openshift#380), so we no longer need to worry about yamllint
vs. Glide.yaml.

CONTRIBUTING.md

@@ -43,6 +43,7 @@ For contributors who want to work up pull requests, the workflow is roughly:
     hack/shellcheck.sh
     hack/tf-fmt.sh -list -check
     hack/tf-lint.sh
+    hack/yaml-lint.sh
     ```
 7. Submit a pull request to the original repository.
 8. The [repo](OWNERS) [owners](OWNERS_ALIASES) will respond to your issue promptly, following [the ususal Prow workflow][prow-review].

hack/yaml-lint.sh

@@ -1,5 +1,4 @@
 #!/bin/sh
-exit 0  # temporarily disable while we work out whether to drop this
 if [ "$IS_CONTAINER" != "" ]; then
   yamllint --config-data "{extends: default, rules: {line-length: {level: warning, max: 120}}}" .
 else