* Add a new ansible role "loadtest_driver" for setting up any instance
  for running load tests.
* Make the existing "locust" role depend on loadtest_driver and pull out
  the redundant tasks.  The "locust" role is outside of the AMI-building
  code path, but we do use it for something else and want to keep common
  tasks shared.
* Make the "jenkins_worker" role depend on the new "loadtest_driver"
  role.
* Create a new ansible playbook for setting up loadtest drivers.
* create a new packer script to build an AMI for loadtest drivers.