enable F811 check for flake8

This change incorporates two cleanups that do not change logic:
- Removes the shadowed unused imports by using the proper oslo.config import
mechanism
- duplicate unit tests have been removed
- duplicate unit test names have been corrected to reflect true test
  nature

Change-Id: Iad7a3373c1f787a2078b932c5f40bb718ad76767
Partial-Bug: 1329017