Be able to override the django sql CONN_MAX_AGE setting.

(cherry picked from commit 1c138599)

CHANGELOG.md

 - Role: edxapp
+  - Added a new EDXAPP_MYSQL_CONN_MAX_AGE, default to 0.  Adjust it to change how long a connection is kept open
+  for reuse before it is closed.
   - Set preload_app to False in gunicorn config for LMS and Studio.
 - Role: xqueue
   - Changed `XQUEUE_RABBITMQ_TLS` default from `true` to `false`.

playbooks/roles/edxapp/defaults/main.yml

@@ -87,6 +87,9 @@ EDXAPP_MYSQL_CSMH_PASSWORD: "{{ EDXAPP_MYSQL_PASSWORD }}"
 EDXAPP_MYSQL_CSMH_HOST: "{{ EDXAPP_MYSQL_HOST }}"
 EDXAPP_MYSQL_CSMH_PORT: "{{ EDXAPP_MYSQL_PORT }}"
 
+# This is Django's default https://docs.djangoproject.com/en/1.8/ref/settings/#conn-max-age
+EDXAPP_MYSQL_CONN_MAX_AGE: 0
+
 EDXAPP_MYSQL_HOST: 'localhost'
 EDXAPP_MYSQL_PORT: '3306'
 
@@ -725,6 +728,7 @@ edxapp_databases:
     PASSWORD: "{{ EDXAPP_MYSQL_REPLICA_PASSWORD }}"
     HOST: "{{ EDXAPP_MYSQL_REPLICA_HOST }}"
     PORT: "{{ EDXAPP_MYSQL_REPLICA_PORT }}"
+    CONN_MAX_AGE: "{{ EDXAPP_MYSQL_CONN_MAX_AGE }}"
   default:
     ENGINE: 'django.db.backends.mysql'
     NAME: "{{ EDXAPP_MYSQL_DB_NAME }}"
@@ -733,6 +737,7 @@ edxapp_databases:
     HOST: "{{ EDXAPP_MYSQL_HOST }}"
     PORT: "{{ EDXAPP_MYSQL_PORT }}"
     ATOMIC_REQUESTS: True
+    CONN_MAX_AGE: "{{ EDXAPP_MYSQL_CONN_MAX_AGE }}"
   student_module_history:
     ENGINE: 'django.db.backends.mysql'
     NAME: "{{ EDXAPP_MYSQL_CSMH_DB_NAME }}"
@@ -740,6 +745,7 @@ edxapp_databases:
     PASSWORD: "{{ EDXAPP_MYSQL_CSMH_PASSWORD }}"
     HOST: "{{ EDXAPP_MYSQL_CSMH_HOST }}"
     PORT: "{{ EDXAPP_MYSQL_CSMH_PORT }}"
+    CONN_MAX_AGE: "{{ EDXAPP_MYSQL_CONN_MAX_AGE }}"
 
 edxapp_generic_auth_config:  &edxapp_generic_auth
   EVENT_TRACKING_SEGMENTIO_EMIT_WHITELIST: "{{ EDXAPP_EVENT_TRACKING_SEGMENTIO_EMIT_WHITELIST }}"