PlantUML jar and java locations should be configurable

We're about to merge changes to make plantuml be headless. This requires running plantuml directly from the jar, not the wrapper script. Also, this means invoking Java directly. The location of the Java bytecode interpreter should also be configurable: at minimum the JAVA_HOME environment variable should be obeyed.

These are necessary, at minimum, for portability beyond Debian Linux.

• Honour JAVA_HOME
• make the paths command line options to docgen, with default values that are incorrect
• ensure tests fail with defaults
• ensure tests pass if options are used to provide correct paths for Debian
• change defaults to be correct for Debian
• ensure tests pass
• add test for failure to run if path is incorrect
• ensure that test passes