This patch adds the basic files and configs in order to enable the
keystone tempest plugin interface using tempest-plugin-cookiecutter.
Since we are adding them inside keystone's repository, they can be
installed alongside keystone and to run the tests (when we have one)
we simply use `testr run keystone_tempest_plugin` in tempest.
For more details about the tempest plugin interface see 
Partially-Implements: bp keystone-tempest-plugin-tests