Perform package upgrades prior to zuul cluster node reboots

This serves two purposes. The first is that not all packages are updated
by unattended-upgrades beacuse it may not be safe to upgrade packages
while services are running. We should be safe in this situation because
we've gracefully stopped services and can proceed with package updates.
The other is unattended upgrades runs daily which means we could end up
almost 24 hours out of date prior to rebooting. This ensures we have the
latest and greatest packages installed just prior to rebooting.

Change-Id: Id351b5478e925ed1b4fbb6b3e27f2c0b6af8b897

playbooks/zuul_reboot.yaml

@@ -13,6 +13,10 @@
       include_role:
         name: zuul-executor
         tasks_from: graceful
+    - name: Upgrade executor server packages
+      apt:
+        update_cache: yes
+        upgrade: yes
     - name: Reboot the executor server
       reboot:
     - name: Start the executor
@@ -28,6 +32,10 @@
       include_role:
         name: zuul-merger
         tasks_from: graceful
+    - name: Upgrade merger server packages
+      apt:
+        update_cache: yes
+        upgrade: yes
     - name: Reboot the merger server
       reboot:
     - name: Start the merger
@@ -49,6 +57,10 @@
       include_role:
         name: zuul-web
         tasks_from: stop
+    - name: Upgrade scheduler server packages
+      apt:
+        update_cache: yes
+        upgrade: yes
     - name: Reboot the scheduler server
       reboot:
     - name: Start the scheduler process