Set of fixes to make the provisioning of DO999 and DO280 courses
compatible with the upstream code.

* Define the Python 3 virtual environment as the Python interpreter for
  the provisioning (ocp4-workshop)
* Set a default value for the 'aws_instance_wait_for_stop' variable in
  the ocp4-workshop role
* Add the boto packages as requirements for the virtual environment
* Fix an issue in the 'lets-encrypt-certs-certbot' role that would
  prevent the installation of the certificates
* Replace the usage of the 'package' module with 'yum' to make the role
  compatible with RHEL 7.X machines
* Replace the usage of the 'firewalld' module with 'shell' to make the
  role compatible with RHEL 7.X machines
* Add boto pre-requisites in the 'remove_workload.yml' playbook
* Force the issue of the let's encrypt cert (WIP)