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/134392/

Change-Id: I5439c587efc64ef0ec3672621eb39c42b2309803
Merge "Add rally job related files"