Don't import module "etcd3gw" if etcd3gw backend is not used.
If module etcd3gw is not required for used backend, then no need to have it installed and imported. Otherwise we always have ugly log error: Could not load 'oslo_cache.etcd3gw': No module named 'etcd3gw': ModuleNotFoundError: No module named 'etcd3gw Closes-Bug: 1928318 Change-Id: Icbe6dc3e93b4d2fec1ceb88366027294e49d7032