Jenkins: Move away from JNLP-based agents-to-controller connection

We may need to switch from JNLP-based agents-to-controller to something else:

• Web Start is not included in Java 11.
• Jenkins is eventually moving to Java 11, once it's upgraded to Buster or Bullseye.

Some options:

• SSH Build Agents (controller-to-agents).
• WebSockets, currently beta (agents-to-controller).