gce/upgrade.sh: Prompt if etcd version is unspecified.

We shouldn't upgrade etcd without first warning the user that some etcd
version transitions can't be undone. We don't know what version the user
currently has, so we require either an explicit version and image, or an
interactive acknowledgement of this caveat.

This is modeled after the STORAGE_MEDIA_TYPE prompt just above.

test/e2e/framework/nodes_util.go

@@ -78,6 +78,10 @@ func masterUpgradeGCE(rawV string, enableKubeProxyDaemonSet bool) error {
 			"TEST_ETCD_VERSION="+TestContext.EtcdUpgradeVersion,
 			"STORAGE_BACKEND="+TestContext.EtcdUpgradeStorage,
 			"TEST_ETCD_IMAGE=3.1.10")
+	} else {
+		// In e2e tests, we skip the confirmation prompt about
+		// implicit etcd upgrades to simulate the user entering "y".
+		env = append(env, "TEST_ALLOW_IMPLICIT_ETCD_UPGRADE=true")
 	}
 
 	v := "v" + rawV