add dashboards for newly supported k8s version (1.15) in gardener
add dashboards for newly supported k8s version (1.15) in gardener

GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid