Configure nofile OS limit for memcached server role

This fix adds a new override `memcached_connections` which is
automatically calculated from the number of memcached
connection limit plus additional 1k to configure the OS nofile limit.
Without proper nofile limit configuration, memcached will crash
in order to support higher parallel connection TCP/Memcache counts.

Closes-Bug: #1582288

Change-Id: I558ba329b7247cdfcf660e392c3ae0c4a0d65a53
(cherry picked from commit 5c2d8c55ec)

defaults/main.yml

@@ -31,6 +31,7 @@ memcached_listen: "127.0.0.1"
 memcached_log: /var/log/memcached.log
 memcached_connections: 1024
 memcached_threads: 4
+memcached_file_limits: "{{ memcached_connections | int + 1024 }}"
 
 memcached_apt_packages:
   - memcached

releasenotes/notes/memcached_server-add-nofile-setting-504e0c50e10a4ea6.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - The openstack-ansible-memcached_server role includes
+    a new override,`memcached_connections` which is
+    automatically calculated from the number of memcached
+    connection limit plus additional 1k to configure
+    the OS nofile limit. Without proper nofile limit
+    configuration, memcached will crash in order to support
+    higher parallel connection TCP/Memcache counts.

tasks/memcached_config.yml

@@ -36,3 +36,19 @@
   notify: Restart memcached
   tags:
     - memcached-config
+
+- name: Configure soft file limits
+  lineinfile:
+    dest: "/etc/security/limits.conf"
+    regexp: "^memcache.*soft.*nofile.*"
+    state: "present"
+    line: "memcache\tsoft\tnofile\t\t{{ memcached_file_limits }}"
+    insertbefore: "^# End of file"
+
+- name: Configure hard file limits
+  lineinfile:
+    dest: "/etc/security/limits.conf"
+    regexp: "^memcache.*hard.*nofile.*"
+    state: "present"
+    line: "memcache\thard\tnofile\t\t{{ memcached_file_limits }}"
+    insertbefore: "^# End of file"