Check if glance_upgrading is defined before using it

glance_upgrading variable is defined only during rolling upgrades, but
its value is checked on every call to "Restart glance-api container" handler.

Closes-Bug: #1826511
Change-Id: Idc95306c50a09666ac0a12e975307a42aef9e352

ansible/roles/glance/handlers/main.yml

@@ -26,5 +26,5 @@
       or glance_swift_conf.changed | bool
       or policy_overwriting.changed | bool
       or glance_api_container.changed | bool
-      or glance_upgrading | bool
+      or ( glance_upgrading is defined and glance_upgrading | bool )
       or ( glance_api_ceph_files is defined and glance_api_ceph_files.changed | bool )