In EC2 deployment, Allow users to set download jars into the lib/xd directory

In cases where the deployment requires jars that can not be included with the distribution, the user should be able to pull a jar from a http site and place it in lib/xd.

The use case is that when we removed the mysql jar from the distribution, the CI tests could not start the XD instances on EC2 without it. It was suggested that we use the postgresql instead, but decided to continue the use of mysql for acceptance tests.