Avoid undefined variable glusterfs_heketi_user_key

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1607513

roles/openshift_storage_glusterfs/tasks/glusterfs_common.yml

@@ -185,7 +185,9 @@
 - name: Generate heketi user key
   set_fact:
     glusterfs_heketi_user_key: "{{ 32 | lib_utils_oo_generate_secret }}"
-  until: "glusterfs_heketi_user_key != glusterfs_heketi_admin_key"
+  until:
+  - glusterfs_heketi_user_key is defined
+  - glusterfs_heketi_user_key != glusterfs_heketi_admin_key
   delay: 1
   retries: 10
   when: