migrate testing to ubuntu focal
migrate testing to ubuntu focal

As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).

Fixing:
- bug#1886296
Bump the pyflakes to 2.1.1 as min version to run pep8 jobs
on py3.8 which is default python vesion in ubuntu focal.

Story: #2007865
Task: #40225

[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal.html
Change-Id: Ib15ad0f23fdf6cba4583790c440bf8ecb75a733d

openstack/trove-dashboard,openstack/trove-dashboard,openstack/trove-dashboard,openstack/trove-dashboard