NFVBENCH-216 Fix broken flake8 tests in CI

- remove version constraints on the "hacking" python package (flake8
  plugin that enforces openstack coding style) because this was the root
  cause holding flake8 in the past.

- require flake8>=3.3.0 to have python 3.6 support

- ignore all the new errors introduced by newer versions of the tools.
  The point here is to have flake8 back with equal functionality.  If
  wanted, some of the new error codes could be re-enabled, but this would
  require to fix the code and could lead to accidental regressions.

- align minimum package version numbers in requirements-dev.txt and
  test-requirements.txt

Signed-off-by: Gwenael Lambrouin <gwenael.lambrouin@orange.com>
Change-Id: I2e85aded58f1b2b9b5e750dba66ac68b13e150a5