Enable minimum CentOS repositories

The CentOS CI images have many yum repositories enabled by default.
This can cause package conflicts in gate jobs and it doesn't match
what a basic CentOS system would look like for an OpenStack-Ansible
deployer.

This patch disables all repositories for the ansible-role-jobs and
only enables base, epel, and updates. This allows individual roles
to add extra repositories when they need them.

Related-Bug: 1708493
Change-Id: I2ac5a9c463991e4668a3e7608831c1944377e85e

jenkins/jobs/ansible-role-jobs.yaml

@@ -77,13 +77,20 @@
           template-name: "{template-name}"
       - zuul-git-prep-upper-constraints
       - shell: |
-          # EPEL must be enabled because some deps from bindep.txt are
-          # otherwise not available
+          # NOTE(mhayden): The CentOS CI image has many repositories enabled by
+          # default that can cause package conflicts. We must disable all of
+          # them here and only enable base, updates, and epel.
           if [[ -e /usr/bin/yum ]] && [[ -x /usr/bin/yum-config-manager ]]; then
+            sudo yum-config-manager --disable \*
+            sudo yum-config-manager --enable base
             sudo yum-config-manager --enable epel
+            sudo yum-config-manager --enable updates
           fi
       - install-distro-packages
       - shell: |
+          # NOTE(mhayden): EPEL is no longer required after installing distro
+          # packages with bindep. Individual roles may re-enable EPEL if they
+          # need it for their package installation tasks.
           if [[ -e /usr/bin/yum ]] && [[ -x /usr/bin/yum-config-manager ]]; then
             sudo yum-config-manager --disable epel
           fi