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: I2d0b500f002e457abcb1d5fe96bf554f96e5700e

defaults/main.yml

@@ -134,6 +134,7 @@ aodh_pip_packages:
   - gnocchiclient
   - osprofiler
   - PyMySQL>=0.6.2
+  - pymemcache
   - python-memcached
   - sqlalchemy>=0.9.7
   - sqlalchemy-utils
@@ -144,6 +145,8 @@ aodh_user_pip_packages: []
 aodh_optional_oslomsg_amqp1_pip_packages:
   - oslo.messaging[amqp1]
 
+aodh_memcached_servers: "{{ memcached_servers }}"
+
 aodh_alarm_notifier_init_overrides: {}
 aodh_alarm_evaluator_init_overrides: {}
 aodh_api_init_overrides: {}

templates/aodh.conf.j2

@@ -29,7 +29,7 @@ username = {{ aodh_service_user_name }}
 password = {{ aodh_service_password }}
 region_name = {{ keystone_service_region }}
 
-memcached_servers = {{ memcached_servers }}
+memcached_servers = {{ aodh_memcached_servers }}
 
 token_cache_time = 300