Update rally job files

Rename rally-scenarios/ to rally-jobs/ Because it makes much more sense
to call directory with jobs files rally-jobs

Add pretty README files, that describes how to write plugins and use
extra dir

Update main README.rst file that describes what the hell is this=)

Add cinder-fakevirt.yaml that will be required after this infra patch
is merged: https://review.openstack.org/#/c/135137/

Change-Id: Ic7e5e8a99fc1385638fb187a04475a42eedcfeb2
Merge "Update rally job files"