tests/vm/centos: fix centos build target

To be able to run the docker tests centos has here we have to install
python3 as well as the basic tools.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>