Allow overriding etcd version in tests

... so that we can try multiple etcd versions more easily.

Change-Id: I2ac707e559252fa18512a28ae7491ec60e73f791

setup-etcd-env.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 set -eux
 if [ -z "$(which etcd)" ]; then
-    ETCD_VERSION=3.4.27
+    ETCD_VERSION=${ETCD_VERSION:-3.4.27}
     case `uname -s` in
         Darwin)
             OS=darwin