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