Prevent "No handlers could be found..." logging

To prevent this message from being logged during unit tests runs
or heat-manage calls, the following changes have been made:
* Make the instance_users deprecation warning use "warnings" and
  move to engine init
* patch out warnings for engine tests
* do not log barbican or marconi if importing their clients fails

Change-Id: Icc4be26bf334a6ce31ad3b8fe9fd244cd9e37be2
Closes-Bug: #1333408
Merge "Prevent "No handlers could be found..." logging"