Add the rally-tool script

The rally-tool script is a Bash script that helps in the automation of
some high level actions performed with Rally.
In this initial version, the actions that executes are the following:
* Create a local custom Rally repository using the main Rally repository
  and then merging defined Rally commits that are under review.
* Install Rally locally and add some user-configured task configuration
  files.
* Use an existing OpenStack deployment and run a defined task against
  it.