Update rally-jobs files

*) Rename rally-scenarios that is quite misleading to rally-jobs.
   rally-jobs makes much more sense, cause it actually contains files
   related to rally job

*) Update rally-jobs/README.rst to add more info

*) Update rally-jobs/plugins/README.rst to expaling plugins

*) Add new directory rally-jobs/extra, this directory is copy pasted
   in gates and can be used for files that are required by some of
   benchmarks

Change-Id: I6d0c0435a4bb4658ddf4adb871bc36ab8c157f3e
Merge "Update rally-jobs files"