Folder-level credentials are not displayed

Similar to a comment in https://github.com/jenkinsci/google-kubernetes-engine-plugin/issues/79#issuecomment-500012467 which is now closed.

Only credentials created at the System Level is rendered under the credentials list.

Can we add support for credentials associated at the folder level? Basically, I would like to eliminate the chance a pipeline may end up deploying a workload to the wrong cluster/project.

[View source](https://github.com/jenkinsci/google-kubernetes-engine-plugin/blob/google-kubernetes-engine-0.6.3/src/main/java/com/google/jenkins/plugins/k8sengine/KubernetesEngineBuilder.java#L378)

Start with src/main/java/com/google/jenkins/plugins/k8sengine/KubernetesEngineBuilder.java around line 378, then review the related comment in issue #79. Trace how the credentials list is assembled and determine how folder-level credentials should appear. Done means credentials associated with a folder are displayed so a pipeline can select the intended cluster or project.

Written by the indexing model from the issue text.