.gitlab-ci.yml: fix vm image jobs

.gitlab-ci.yml: fix vm image jobs

1) use CentOS 7 hosts to build VM images: CentOS 8 is not supported as a
build host according to diskimage-builder documentation:
https://docs.openstack.org/diskimage-builder/latest/user_guide/supported_distros.html
(page checked on 2022-02-08)

2) add missing packages for diskimage-builder: qemu-img kpartx sudo
e2fsprogs

3) use "python3 -m venv" instead of virtualenv (else the pipeline fails
because of https://github.com/pypa/virtualenv/issues/1029)

Change-Id: I14daf76dded1f230e12c47218ddcb1fe9029bcd8
Signed-off-by: Gwenael Lambrouin <gwenael.lambrouin@orange.com>