With Kubernetes on can define storage classes with provisioners. How does one find which provisioners are installed and available in the cluster?
Inspecting the storage classes will reveal which provisioners are already in use, but not whether there are more available.
A provisioner does not necessarily need to run in the cluster, e.g. the provisioner for an external storage appliance just connects to the cluster api server and watches for new persistent volume requests created with a storage class bound to its provisioner name. This is why as of Kubernetes 1.7 there is no intended universal way to see if a storage classes provisioner is actually available or not.