Add functional API tests for cron-triggers

- Added API tests which check main api calls to cron-trigger endpoint
- Fixed tearDown method (there was a problem when item array
had a few elements, not all of them were deleted)

Partly implements: blueprint mistral-triggers-functional-tests

Change-Id: Ibba7cd03e6ccdf06c76ea4ea161542b954faee0f
Merge "Add functional API tests for cron-triggers"