[dhcp] Make use of handlers to restart the server only when needed

roles/isc-dhcp-server/handlers/main.yml

+---
+- name: restart dhcp server
+  systemd:
+    name: isc-dhcp-server
+    state: restarted
+    enabled: true

roles/isc-dhcp-server/tasks/main.yml

@@ -13,9 +13,5 @@
     src: dhcp/dhcpd.conf.j2
     dest: /etc/dhcp/dhcpd.conf
     mode: 0600
-
-- name: Ensure that isc-dhcp-server is started
-  systemd:
-    name: isc-dhcp-server
-    state: started
-    enabled: true
+  notify:
+    - restart dhcp server