upstream CI: removed all CentOS 8 support.

CentOS 8 images are not supported anymore, and we are using CentOS 8
Stream images.

This patch removes all configuration for CentOS 8 and updates test
README to point to the available container images.

molecule/centos-8-build/molecule.yml

@@ -1,19 +0,0 @@
----
-driver:
-  name: docker
-platforms:
-  - name: centos-8-build
-    image: "centos:centos8"
-    pre_build_image: true
-    hostname: ipaserver.test.local
-    dns_servers:
-      - 8.8.8.8
-    volumes:
-      - /sys/fs/cgroup:/sys/fs/cgroup:ro
-    command: /usr/sbin/init
-    privileged: true
-provisioner:
-  name: ansible
-  playbooks:
-    prepare: ../resources/playbooks/prepare-build.yml
-prerun: false

molecule/centos-8/molecule.yml

@@ -1,19 +0,0 @@
----
-driver:
-  name: docker
-platforms:
-  - name: centos-8
-    image: quay.io/ansible-freeipa/upstream-tests:centos-8
-    pre_build_image: true
-    hostname: ipaserver.test.local
-    dns_servers:
-      - 127.0.0.1
-    volumes:
-      - /sys/fs/cgroup:/sys/fs/cgroup:ro
-    command: /usr/sbin/init
-    privileged: true
-provisioner:
-  name: ansible
-  playbooks:
-    prepare: ../resources/playbooks/prepare.yml
-prerun: false

molecule/default

@@ -1 +1 @@
-centos-8
+fedora-latest