Clear expired Django sessions regularly from DB

A random minute in the 21st hour UTC, daily, was chosen arbitrarily.
That it is run on a single arbitrary host from the set was done to
prevent unnecessary work being done until Bug #1424705 provides us with
a cleaner solution.

Adjusted to run once per day just to be sure that the operation doesn't
lock the table for a meaningful duration.

Making this configurable or tunable does not seem
to be necessary at this time.

Change-Id: I87164926a4d6a65d3c9f733d625a2d4af91c2597
Closes-Bug: #1466126

playbooks/roles/os_horizon/tasks/horizon_db_setup.yml

@@ -46,3 +46,14 @@
   tags:
     - horizon-db-sync
     - horizon-setup
+
+- name: Register DB session cleanup cron
+  cron:
+    name: "Clear out expired sessions"
+    minute: "{{ 58 | random(start=2) }}"
+    hour: 21
+    job: "/usr/local/bin/horizon-manage.py clearsessions"
+    user: "{{ horizon_system_user_name }}"
+    state: present
+  tags:
+    - horizon-db-setup