Add rally job related files

* Add README.rst file with useful description
* Add extra and plugins directory with corresponding README files
* Add rally task designate.yaml that will be actually run in gates

Infra related patch is here:
https://review.openstack.org/#/c/135132/

Change-Id: Ida3c3aa24f63592f9c4bd7143d30377fddd021a6
Merge "Add rally job related files"