zuul-worker: no selinux python2 libs on CentOS 8

There are no libselinux bindings for Python 2 on CentOS 8 [1].  The
Python 3 packages are there as part of the base install.

These are required for Ansible to run correctly.  We will need to set
ansible_python_interpreter to python3 for CentOS 8 until
auto-detection is complete with [2].

[1] https://bugs.centos.org/view.php?id=16458
[2] https://review.opendev.org/682797

Change-Id: I52eef3069fa8699cc72b423d476c92b339600c23

nodepool/elements/zuul-worker/pkg-map

@@ -1,4 +1,11 @@
 {
+  "release": {
+    "centos": {
+      "8": {
+        "libselinux-python": ""
+       }
+    }
+  },
   "family": {
     "debian": {
       "apt-transport-https": "apt-transport-https",