Make horizon DB management cron task idempotent

Use a seed in the glance cache management cron task's random
filter so that each host's results are idempotent when re-running.
This prevents the constant changing of the crontab whenever the
playbook is run.

Change-Id: I441d5d30e06f24a1149926f9068e7e4838b3a723

tasks/horizon_db_setup.yml

@@ -48,7 +48,7 @@
 - name: Register DB session cleanup cron
   cron:
     name: "Clear out expired sessions"
-    minute: "{{ 58 | random(start=2) }}"
+    minute: "{{ 58 | random(seed=inventory_hostname, start=2) }}"
     hour: 21
     job: "{{ horizon_manage }} clearsessions"
     user: "{{ horizon_system_user_name }}"