Stagger package upgrades across the available hosts

roles/base/tasks/main.yml

 ---
 
-- name: Upgrade existing packages
-  apt:
-    update_cache: true
-    upgrade: true
-
 - name: Set hostname
   copy:
     dest: /etc/hostname

site.yml

 ---
 
+# First update packages on all hosts, staggering execution so as to
+# not restart all radiod nodes at once on package upgrades.
+- hosts: all
+  tasks:
+    - name: Upgrade packages
+      apt:
+        update_cache: true
+        upgrade: true
+  serial:
+    - 1
+    - 1
+    - "100%"
+
 - hosts: all
   roles:
     - base