Refactor memcached_servers

This patch aims to add a prefix for memcached_server
on each role to give the ability for deployers to
override the location of memcached cluster. I.e users
wants to create a single memcached cluster with k8s
for each service.

We also add pymemcache based on [1]

[1] https://review.opendev.org/711429

Change-Id: I59fd6bd2e9557e8806e74bae8980ce322dcc748c

defaults/main.yml

@@ -136,7 +136,7 @@ horizon_session_engine: 'django.contrib.sessions.backends.cache'
 horizon_session_caches:
   default:
     BACKEND: 'django.core.cache.backends.memcached.MemcachedCache'
-    LOCATION: "{{ memcached_servers.split(',') }}"
+    LOCATION: "{{ horizon_memcached_servers.split(',') }}"
 horizon_session_timeout: 1800
 
 ## Horizon Help URL Path
@@ -390,8 +390,12 @@ horizon_listen_ports:
 
 horizon_pip_packages:
   - horizon
+  - pymemcache
   - python-memcached
 
+# Memcached override
+horizon_memcached_servers: "{{ memcached_servers }}"
+
 # Specific pip packages provided by the user
 horizon_user_pip_packages: []