Fix sphinx docs generation

After members section was made optional, it should be also optional
in sphinx.yml

Change-Id: Id4fe526f240048144e25016aaa54f88be81df142
diff --git a/gnocchi/meta/sphinx.yml b/gnocchi/meta/sphinx.yml
index d2a8235..1e35a98 100644
--- a/gnocchi/meta/sphinx.yml
+++ b/gnocchi/meta/sphinx.yml
@@ -43,7 +43,9 @@
         indexer_url:
           name: "URL to indexer backend"
           value: {{ cfg.database.engine }}+pymysql://{{ cfg.database.user }}:{{ cfg.database.password }}@{{ cfg.database.host }}/{{ cfg.database.name }}{%- if cfg.database.get('ssl',{}).get('enabled',False) %}?ssl_ca={{ cfg.database.ssl.get('cacert_file', cfg.cacert_file) }}{% endif %}
+      {%- if server.get('cache', {}).members is defined %}
         cache_memcached_servers:
           name: "Memcached servers for token cache"
           value: {% for member in server.cache.members %}{{ member.host }}:{{ member.port }}{% if not loop.last %},{% endif %}{% endfor %}
+      {%- endif %}
   {%- endif %}