Uses explicit imports for _

Previously `_` was monkeypatched in tests/core.py and bin/keystone-*.
This meant that if a developer was not running the tests exactly as
the documentation described they would not work. Even importing
certain modules in a interactive Python interpreter would fail unless
keystone.tests was imported first. Monkeypatching was removed and
explicit import for `_` was added.

Co-Authored-By: David Stanek <dstanek@dstanek.com>
Change-Id: I8b25b5b6d83fb873e25a8fab7686babf1d2261fa
Closes-Bug: #1255518
Merge "Uses explicit imports for _"