Use the good configuration directive for memcache servers

Under Pike, both the backend argument and memcache_servers work.
They both end-up with connections to the backend of memcache.

The latter doesn't have the bug when multiple servers are given,
though. So let's use this one exclusively.

This massive template change is for backwards compatibility:
If a deployer has set on of those keystone variables, the servers
used won't change.

Change-Id: I7e29aa0a9055b737dd4cf171c40e2105e4b32e94
Related-Bug: #1743036

defaults/main.yml

@@ -88,11 +88,6 @@ keystone_resource_driver: sql
 
 keystone_bind_address: 0.0.0.0
 
-## Memcached servers used within keystone.
-# String or Comma separated list of servers.
-keystone_memcached_servers: 127.0.0.1
-keystone_memcached_max_compare_and_set_retry: 16
-
 ## DB info
 keystone_galera_user: keystone
 keystone_galera_database: keystone

templates/keystone.conf.j2

@@ -32,16 +32,32 @@ driver = messagingv2
 transport_url = rabbit://{% for host in keystone_rabbitmq_telemetry_servers.split(',') %}{{ keystone_rabbitmq_telemetry_userid }}:{{ keystone_rabbitmq_telemetry_password }}@{{ host }}:{{ keystone_rabbitmq_telemetry_port }}{% if not loop.last %},{% else %}/{{ keystone_rabbitmq_telemetry_vhost }}{% endif %}{% endfor %}
 {% endif %}
 
+{% set _memcached_servers = '' %}
 {% if keystone_cache_backend_argument is defined %}
+{%   set _memcached_servers_cleaned = keystone_cache_backend_argument | replace('url:','') %}
+{%   set _memcached_servers_listsrc = _memcached_servers_cleaned.split(',') %}
+{%   set _memcached_servers_listdest = [] %}
+{%   for _server in _memcached_servers_listsrc %}
+{%     if ":" not in _server %}
+{%       set _server = _server ~ ':' ~ memcached_port %}
+{%     endif %}
+{%   set _ = _memcached_servers_listdest.append(_server) %}
+{%   endfor %}
+{%   set _memcached_servers = _memcached_servers_listdest | join(',') %}
+{% elif keystone_memcached_servers is defined %}
+{%   set _memcached_servers = keystone_memcached_servers %}
+{% elif memcached_servers is defined %}
+{%   set _memcached_servers = memcached_servers %}
+{% endif %}
+{% if _memcached_servers | length > 0 %}
 [cache]
 backend = dogpile.cache.memcached
-backend_argument = {{ keystone_cache_backend_argument }}
+memcache_servers = {{ _memcached_servers }}
 config_prefix = cache.keystone
 distributed_lock = True
 enabled = true
 {% endif %}
 
-
 [revoke]
 caching = true
 driver = {{ keystone_revocation_driver }}