Refactored Neutron tempest plugin directory structure

* switch from neutron.tests.tempest to neutron_tempest_plugin
* Cleaned up README.rst and setup.cfg
* Use neutron_tempest_plugin as a tempest plugin package
* Fixed gitreview
* Keeping flake8 Ignores in tox.ini as tempest plugin is
  imported from neutron codebase.

Change-Id: I42d389836e72813fdeebc797a577f4a8ac2ee603
85 files changed