upstream CI: Enable CentOS 8 Stream for PR and nightly tests.

Add configuration to build a testing CentOS 8 stream image and to
execute upstream tests using that image in pull requests (Ansible
2.9) and on the nightly tests (all supported Ansible versions).

molecule/c8s-build/Dockerfile

@@ -0,0 +1,30 @@
+FROM quay.io/centos/centos:stream8
+ENV container=docker
+
+RUN rm -fv /var/cache/dnf/metadata_lock.pid; \
+dnf makecache; \
+dnf --assumeyes install \
+    /usr/bin/python3 \
+    /usr/bin/python3-config \
+    /usr/bin/dnf-3 \
+    sudo \
+    bash \
+    systemd \
+    procps-ng \
+    iproute && \
+dnf clean all; \
+(cd /lib/systemd/system/sysinit.target.wants/; for i in *; do [ $i == systemd-tmpfiles-setup.service ] || rm -f $i; done); \
+rm -f /lib/systemd/system/multi-user.target.wants/*;\
+rm -f /etc/systemd/system/*.wants/*;\
+rm -f /lib/systemd/system/local-fs.target.wants/*; \
+rm -f /lib/systemd/system/sockets.target.wants/*udev*; \
+rm -f /lib/systemd/system/sockets.target.wants/*initctl*; \
+rm -f /lib/systemd/system/basic.target.wants/*;\
+rm -f /lib/systemd/system/anaconda.target.wants/*; \
+rm -rf /var/cache/dnf/;
+
+STOPSIGNAL RTMIN+3
+
+VOLUME ["/sys/fs/cgroup"]
+
+CMD ["/usr/sbin/init"]