prow.sh: AllAlpha=true for unknown Kubernetes versions

This ensures that also new, currently unknown alpha gates are enabled
when testing against a future Kubernetes versions. For all currently
known Kubernetes versions we just use the minimal set of alpha gates,
which ensures that we don't miss any of them in our documentation.

prow.sh

@@ -246,8 +246,9 @@ configvar CSI_PROW_E2E_ALPHA "$(get_versioned_variable CSI_PROW_E2E_ALPHA "${csi
 # the failing test for "latest" or by updating the test and not running
 # it anymore for older releases.
 configvar CSI_PROW_E2E_ALPHA_GATES_1_13 'VolumeSnapshotDataSource=true,BlockVolume=true,CSIBlockVolume=true' "alpha feature gates for Kubernetes 1.13"
+configvar CSI_PROW_E2E_ALPHA_GATES_1_14 'VolumeSnapshotDataSource=true' "alpha feature gates for Kubernetes 1.14"
 # TODO: add new CSI_PROW_ALPHA_GATES entry for future Kubernetes releases
-configvar CSI_PROW_E2E_ALPHA_GATES_LATEST 'VolumeSnapshotDataSource=true' "alpha feature gates for latest Kubernetes"
+configvar CSI_PROW_E2E_ALPHA_GATES_LATEST 'AllAlpha=true' "alpha feature gates for latest Kubernetes"
 configvar CSI_PROW_E2E_ALPHA_GATES "$(get_versioned_variable CSI_PROW_E2E_ALPHA_GATES "${csi_prow_kubernetes_version_suffix}")" "alpha E2E feature gates"
 
 # Some tests are known to be unusable in a KinD cluster. For example,