Fix calls to assert_called_once in unit tests

Mock has a method called assert_called_once_with to check that a mock
was called and the arguments it took were as expected. Mock does not
have a method called assert_called_once and calling it just creates a
mock bound to that name. This means that not only is nothing tested
when assert_called_once is used, the tests also don't warn about this.

This commit attempts to address this in two ways:
    - all occurrences of assert_called_once are replaced with a real
      assertion.
    - the hacking check that nova uses to guard against this has been
      copied to cinder's local hacking checks.

Fixing the assert_called_once issues also highlighted other mistakes
in certain tests which were addressed to make the tests pass.

Due to the nature of mock, this issue is also possible if a method is
misspelt or just mistakenly used and so the hacking check is only
addressing one very specific case. That said, it does appear to be a
common mistake and so is worth singling out.

Change-Id: Iedcc3f48d91f7ebd8878ccc3bca3d023503774bd
Closes-Bug: #1394544
Merge "Fix calls to assert_called_once in unit tests"