Check panko.cache.members is defined or not

Change-Id: I0b2074e0ff57864fa107d585c6a9bc79f017157f
diff --git a/panko/files/pike/panko.conf.Debian b/panko/files/pike/panko.conf.Debian
index 55a320b..7420768 100644
--- a/panko/files/pike/panko.conf.Debian
+++ b/panko/files/pike/panko.conf.Debian
@@ -361,7 +361,7 @@
 # undefined, tokens will instead be cached in-process. (list value)
 # Deprecated group/name - [keystone_authtoken]/memcache_servers
 #memcached_servers = <None>
-{%- if server.cache is defined %}
+{%- if server.get('cache', {}).members is defined %}
 memcached_servers = {%- for member in server.cache.members %}{{ member.host }}:{{ member.get('port', '11211') }}{% if not loop.last %},{% endif %}{%- endfor %}
   {%- if server.cache.get('security', {}).get('enabled', False) %}
 memcache_security_strategy = {{ server.cache.security.get('strategy', 'ENCRYPT') }}