infra-pkg-needs: Update pkg-maps for CentOS 8, select chronyd

These images are Python3 only -- no python-dev

The ntp tools are replaced with chrony on CentOS 8, select it instead.
Update the service enablement too.  I have done a quick audit and
I don't believe any of our base job parts rely on ntpdate as such
(except for deprecated devstack-gate; there is actually an
unmerged ancient change! [1]).

[1] https://review.opendev.org/#/c/352674/1/functions.sh

Change-Id: Id2dbda7f114de0be4e4227da179490a17a22eb24

nodepool/elements/infra-package-needs/pkg-map

@@ -1,4 +1,13 @@
 {
+  "release": {
+    "centos": {
+      "8": {
+        "python-dev": "",
+        "ntp": "chrony",
+        "ntpdate": ""
+       }
+    }
+  },
   "distro": {
     "fedora": {
       "python3-dev": "python3-devel",

nodepool/elements/infra-package-needs/post-install.d/80-enable-infra-services

@@ -14,6 +14,8 @@ case "$DIB_INIT_SYSTEM" in
     systemd)
         if [[ $DISTRO_NAME = "ubuntu" || $DISTRO_NAME = "debian" ]]; then
             systemctl enable ntp.service
+        elif [[ $DISTRO_NAME == "centos" && $DIB_RELEASE -ge 8 ]]; then
+            systemctl enable chronyd
         else
             systemctl enable ntpd.service
         fi