Update rally-jobs files

* Rename rally-scenarios directory to rally-jobs. Because actually
  this directory contains files related to job and not scenarios.

* Upadte main README file

* Add readme files to rally-jobs/extra and rally-jobs/plugins

Change-Id: I111c3c3f86ab4886d3a7b66c7bf7c050c1d9ca1a
Merge "Update rally-jobs files"