New cron type for audit interval

This patch set adds cron supporting for audit.

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

watcher/common/utils.py

@@ -16,8 +16,11 @@
 
 """Utilities and helper functions."""
 
+import datetime
 import re
 
+from croniter import croniter
+
 from jsonschema import validators
 from oslo_log import log as logging
 from oslo_utils import strutils
@@ -63,6 +66,15 @@ is_int_like = strutils.is_int_like
 strtime = timeutils.strtime
 
 
+def is_cron_like(value):
+    """Return True is submitted value is like cron syntax"""
+    try:
+        croniter(value, datetime.datetime.now())
+    except Exception as e:
+        raise exception.CronFormatIsInvalid(message=str(e))
+    return True
+
+
 def safe_rstrip(value, chars=None):
     """Removes trailing characters from a string if that does not make it empty