* Use bionic for ussuri build as it's the latest supported release [0][1]. * Switch to centos 8 to run master [1][2]. * Switch default base image to ubuntu:focal. [0] https://governance.openstack.org/tc/reference/runtimes/ussuri.html [1] https://governance.openstack.org/tc/reference/runtimes/victoria.html [2] https://governance.openstack.org/tc/reference/runtimes/train.html Change-Id: I9a70aebd9636e170c6e04398356933668a2bfbba Signed-off-by: Andrii Ostapenko <andrii.ostapenko@att.com>