cloud-setup: let dispatcher script run tool only if service is enabled

We don't want that when the user installs the package, that the
dispatcher script automatically executes the tool. Instead, the user
should use `systemctl enable/disable` to control whether the service
is active (of via the timer).

Hence, let the dispatcher script check whether the service is enabled.

That leads to a different problem, that we need to make it possible for
"nm-cloud-setup.service" to be enabled in the first place. As such, add
a [Install] section and let it be wanted by NetworkManager.service. The
problem with this is that now the tool will run very early, just after
NetworkManager started. At that point, it might not yet have setup
networking. But that should be acceptable, after all, the tool either
fails to fetch meta data that early, or it succeeds. Very likely, it
will by aborted by dispatcher's restart command.

clients/cloud-setup/90-nm-cloud-setup.sh

@@ -2,6 +2,8 @@
 
 case "$2" in
     up|dhcp4-change)
-        exec systemctl --no-block restart nm-cloud-setup.service
+        if systemctl -q is-enabled nm-cloud-setup.service ; then
+            exec systemctl --no-block restart nm-cloud-setup.service
+        fi
         ;;
 esac

clients/cloud-setup/nm-cloud-setup.service.in

@@ -1,5 +1,6 @@
 [Unit]
 Description=Automatically configure NetworkManager in cloud
+After=NetworkManager.service
 
 [Service]
 Type=oneshot
@@ -25,3 +26,6 @@ RestrictNamespaces=yes
 RestrictRealtime=yes
 RestrictSUIDSGID=yes
 SystemCallFilter=@system-service
+
+[Install]
+WantedBy=NetworkManager.service