infra-package-needs: drop ntp for Focal

systemd timesyncd is the default mechanism for timesync on Focal;
let's reduce our modification footprint by not overinstalling ntp or
trying to enable it.

Change-Id: I60e15b9101511e9008159b7a0b63f1b4b3febb96

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

@@ -6,7 +6,13 @@
         "ntp": "chrony",
         "ntpdate": ""
        }
-    }
+    },
+    "ubuntu": {
+      "focal": {
+        "ntp": "",
+        "ntpdate": ""
+      }
+   }
   },
   "distro": {
     "fedora": {

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

@@ -13,7 +13,10 @@ case "$DIB_INIT_SYSTEM" in
         ;;
     systemd)
         if [[ $DISTRO_NAME = "ubuntu" || $DISTRO_NAME = "debian" ]]; then
-            systemctl enable ntp.service
+            # stick with default systemd timesyncd on focal
+            if [[ ${DIB_RELEASE} != 'focal' ]]; then
+                systemctl enable ntp.service
+            fi
         elif [[ $DISTRO_NAME == "centos" && $DIB_RELEASE -ge 8 ]]; then
             systemctl enable chronyd
         else