Install Python 3 libselinux packages for Fedora

Install the Python 3 libselinux packages for Fedora platforms.  I
think this is the right choice; Fedora is a Python-3 only distro so we
shouldn't default to installing the python2 libraries.

This has a practical effect if you're using Ansible with
ansible_python_interpreter=/usr/bin/python3 as it needs these
packages.

There is some small chance of breakage if you're using Ansible still
with Python 2, I guess.  In infra I notice we bring this in with
"zuul-worker" project-config element.  On balance, I think that if you
need the Python 2 packages for some reason, it should be a special
install and not part of redhat-common.

Change-Id: Ibcec0b3660d01b861838c2ae87ca43d98953ce32

diskimage_builder/elements/redhat-common/pkg-map

@@ -1,4 +1,11 @@
 {
+  "distro": {
+    "fedora": {
+      "libselinux-python": "python3-libselinux",
+      "policycoreutils": "python3-policycoreutils",
+      "policycoreutils-python": "policycoreutils-python-utils"
+    }
+  },
   "release": {
     "rhel": {
       "8": {