prow.sh: update to kind 0.9, support Kubernetes 1.19
prow.sh: update to kind 0.9, support Kubernetes 1.19

kind 0.9 adds support for recent Kubernetes releases like 1.19 and
simplifies configuration of feature gates and runtime config.

With Kubernetes 1.19, new feature gates were introduced which might
become relevant for the alpha Prow jobs.

The updated kind release comes with images for different Kubernetes
releases than the one before. To avoid breaking existing jobs,
the script now picks kind images automatically. As an additional
bonus, it then uses images with hash, i.e. it's less likely to break
if those image tags change because of a future kind release.

kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-driver-nfs