Run hacking and flake checks for real

Previous tox configuration didn't run any of hacking or flake8 checks. This
commit enables both. Here are details:
* Added hacking to test-requirements. It'll manage it's own dependencies -
  pep8, pyflakes, and flake8
* Enabled flake8
* Disabled a bunch of hacking and flake rules. They should be fixed in next
  dependend patches
* Removed unused test dependency - pysendfiles

Change-Id: I8dcbd4da6d0ced7e9f6a5a760020cbefa62657e9
Merge "Run hacking and flake checks for real"