Now that we are python3 only, we should move to using the built
in version of mock that supports all of our testing needs and
remove the dependency on the "mock" package.
This completes removal of all references to "import mock",
changing to "from unittest import mock" in fullstack and
functional tests.
Added a hacking check to enforce it in future patches.
Change-Id: Ifcaf1c21bea0ec3c35278e49cecc90a101a82113