tox.ini cleanup
tox.ini cleanup

* Remove requirements.txt from [testenv] deps
  It is automatically done when a pacakge is installed by pbr
* Remove PYTHONHASHSEED=0
  All tests are passed without this
* Drop pylint tox env. pylint is run in pep8 env
* Remove openstack/common from flake8 exclude
* Remove flake8 builtins. _ now should be consumed from oslo i18n wrapper
* Remove show-tree to reduce the output size
* Remove unused flake8 ignore
* Delete unnecessary commented-out line.

Change-Id: I5b36c8fd08ad10be4ff7d2f424d4b363a279ea0f

openstack/networking-nec,stackforge/networking-nec,openstack/networking-nec,stackforge/networking-nec