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
Expose nullable fields properties

Some fields are not required parameters, therfore this patch allows them
to be None.

This has been done for images in a previous patch. This patch does the
same thing for tasks.

This will allow the client to have attributes with value `None` and
it'll make it consistent across responses and with regards to the API.

NOTE: A follow-up patch will enable None values to be returned by the
API.

Change-Id: I694cc144375c76ad8aaab495fc2e3ac5ed84ec2f
Merge "Update rally-jobs files"
Merge "Expose nullable fields properties"