prow.sh: fix E2E suite for Kubernetes >= 1.18

It used to be necessary to override from where the E2E suite came on a
case-by-case basis (initially, testing was using a more recent suite
against an older Kubernetes). This should never become necessary again
and the lack of a specific entry for 1.18 already had the unintended
effect that Kubernetes 1.18 was tested with the suite from master, so
overall it is better to always use the E2E suite which matches
Kubernetes.

prow.sh

@@ -218,17 +218,18 @@ configvar CSI_PROW_DRIVER_CANARY "${CSI_PROW_HOSTPATH_CANARY}" "driver image ove
 # all generated files are present.
 #
 # CSI_PROW_E2E_REPO=none disables E2E testing.
-# TOOO: remove versioned variables and make e2e version match k8s version
-configvar CSI_PROW_E2E_VERSION_1_15 v1.15.0 "E2E version for Kubernetes 1.15.x"
-configvar CSI_PROW_E2E_VERSION_1_16 v1.16.0 "E2E version for Kubernetes 1.16.x"
-configvar CSI_PROW_E2E_VERSION_1_17 v1.17.0 "E2E version for Kubernetes 1.17.x"
-# TODO: add new CSI_PROW_E2E_VERSION entry for future Kubernetes releases
-configvar CSI_PROW_E2E_VERSION_LATEST master "E2E version for Kubernetes master" # testing against Kubernetes master is already tracking a moving target, so we might as well use a moving E2E version
-configvar CSI_PROW_E2E_REPO_LATEST https://github.com/kubernetes/kubernetes "E2E repo for Kubernetes >= 1.13.x" # currently the same for all versions
-configvar CSI_PROW_E2E_IMPORT_PATH_LATEST k8s.io/kubernetes "E2E package for Kubernetes >= 1.13.x" # currently the same for all versions
-configvar CSI_PROW_E2E_VERSION "$(get_versioned_variable CSI_PROW_E2E_VERSION "${csi_prow_kubernetes_version_suffix}")"  "E2E version"
-configvar CSI_PROW_E2E_REPO "$(get_versioned_variable CSI_PROW_E2E_REPO "${csi_prow_kubernetes_version_suffix}")" "E2E repo"
-configvar CSI_PROW_E2E_IMPORT_PATH "$(get_versioned_variable CSI_PROW_E2E_IMPORT_PATH "${csi_prow_kubernetes_version_suffix}")" "E2E package"
+tag_from_version () {
+    version="$1"
+    shift
+    case "$version" in
+        latest) echo "master";;
+        release-*) echo "$version";;
+        *) echo "v$version";;
+    esac
+}
+configvar CSI_PROW_E2E_VERSION "$(tag_from_version "${CSI_PROW_KUBERNETES_VERSION}")"  "E2E version"
+configvar CSI_PROW_E2E_REPO "https://github.com/kubernetes/kubernetes" "E2E repo"
+configvar CSI_PROW_E2E_IMPORT_PATH "k8s.io/kubernetes" "E2E package"
 
 # csi-sanity testing from the csi-test repo can be run against the installed
 # CSI driver. For this to work, deploying the driver must expose the Unix domain