Update oslo.cache to 1.20.0
Update oslo.cache to 1.20.0

Change-Id: Ib8676e2d9fcf8bc547b08e8f65c863e717781b9b
Depends-on: I06af9f3268db6517d4e3a7759af70d39e764898b

openstack/rpm-packaging,openstack/rpm-packaging