random filter: added optional 'seed' parameter

implements #15621

docsite/rst/playbooks_filters.rst

@@ -168,6 +168,11 @@ Get a random number from 1 to 100 but in steps of 10::
     {{ 100 |random(1, 10) }}    => 31
     {{ 100 |random(start=1, step=10) }}    => 51
 
+It's also possible to initialize random number generator from seed. This way, you can create random-but-idempotent
+numbers (new in version 2.3)::
+
+    {{ 59 |random(seed=inventory_hostname) }} * * * * root /script/from/cron
+
 
 Shuffle Filter
 --------------

lib/ansible/plugins/filter/core.py

@@ -31,7 +31,7 @@ import crypt
 import hashlib
 import string
 from functools import partial
-from random import SystemRandom, shuffle
+from random import Random, SystemRandom, shuffle
 from datetime import datetime
 import uuid
 
@@ -199,8 +199,11 @@ def from_yaml(data):
     return data
 
 @environmentfilter
-def rand(environment, end, start=None, step=None):
-    r = SystemRandom()
+def rand(environment, end, start=None, step=None, seed=None):
+    if seed is None:
+        r = SystemRandom()
+    else:
+        r = Random(seed)
     if isinstance(end, (int, long)):
         if not start:
             start = 0