CentOS 8 CI (#5842)

* requirements.txt: Bump versions

Ansible 2.8+ allow ansible_python_interpreter autodetection

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

* tests: do not force ansible_python_interpreter

we do not expect people to set ansible_python_interpreter, so we should not set it in the CI

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

* Add CentOS 8 Calico to CI

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>