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: Ibf64fc04bd9b4359de902a92aa39cfc2923c8823

defaults/main.yml

@@ -93,10 +93,14 @@ placement_role_name: admin
 placement_pip_packages:
   - openstack-placement
   - PyMySQL
+  - pymemcache
   - python-memcached
   - cryptography
   - systemd-python
 
+# Memcached override
+placement_memcached_servers: "{{ memcached_servers }}"
+
 ## Service Name-Group Mapping
 placement_services:
   placement-api:

templates/placement.conf.j2

@@ -14,7 +14,7 @@ project_name = {{ placement_service_project_name }}
 username = {{ placement_service_user_name }}
 password = {{ placement_service_password }}
 region_name = {{ keystone_service_region }}
-memcached_servers = {{ memcached_servers }}
+memcached_servers = {{ placement_memcached_servers }}
 memcache_security_strategy = ENCRYPT
 memcache_secret_key = {{ memcached_encryption_key }}