New cron type for audit interval

This patch set adds cron supporting for audit.

Implements: blueprint cron-based-continuous-audits
Change-Id: I8570bebb13332dfba80185e912aeda45b6b4cd70

requirements.txt

@@ -9,6 +9,7 @@ keystoneauth1>=2.21.0 # Apache-2.0
 jsonschema!=2.5.0,<3.0.0,>=2.0.0 # MIT
 keystonemiddleware>=4.12.0 # Apache-2.0
 lxml!=3.7.0,>=2.3 # BSD
+croniter>=0.3.4 # MIT License
 oslo.concurrency>=3.8.0 # Apache-2.0
 oslo.cache>=1.5.0 # Apache-2.0
 oslo.config!=4.3.0,!=4.4.0,>=4.0.0 # Apache-2.0