Check cache.members is defined or not

Change-Id: Iac132929383d9f01aa1ecc40f04a841ffed9738a
diff --git a/oslo_templates/files/queens/keystonemiddleware/_auth_token.conf b/oslo_templates/files/queens/keystonemiddleware/_auth_token.conf
index d57d9a7..755a991 100644
--- a/oslo_templates/files/queens/keystonemiddleware/_auth_token.conf
+++ b/oslo_templates/files/queens/keystonemiddleware/_auth_token.conf
@@ -89,7 +89,7 @@
 # undefined, tokens will instead be cached in-process. (list value)
 # Deprecated group/name - [keystone_authtoken]/memcache_servers
 #memcached_servers = <None>
-{%- if _data.cache is defined %}
+{%- if _data.get('cache', {}).members is defined %}
 memcached_servers={%- for member in _data.cache.members %}{{ member.host }}:11211{% if not loop.last %},{% endif %}{%- endfor %}
 
 # In order to prevent excessive effort spent validating tokens, the middleware