Add croniter dependency

A cron library, will be used in Ceilometer alarms.

License is MIT, lower bound is based on the added Py3 support.

Is also used by Mistral: https://github.com/stackforge/mistral/blob/master/requirements.txt

Change-Id: I9b818c171d9ca2e4e8afd8058769be471b2ff800
Blueprint: time-constrained-alarms
Merge "Add croniter dependency"