Revert "[goal] Migrate testing to ubuntu focal"
Revert "[goal] Migrate testing to ubuntu focal"

This reverts commit 3bb7b71e1afb378d47b449e9ba8ff8f633abe7d9.

https://github.com/gnocchixyz/gnocchi/issues/1069 was fixed

Change-Id: I3277ed13b4b343c553a3c21eab2187bb8abd4251

openstack/ceilometer,openstack/ceilometer