MDL-41421 tool_generator: Adding a test plan generator to feed JMeter

Also changing course_backend.php coding style
according to codechecker.
MDL-41421 tool_generator: Adding a CLI tool to generate the test plan
MDL-41421 tool_generator: Using default values in the test plan

Using optional properties allows us to only set the default
value, so if nothing else is specified when running jmeter the
default value will be used as it is being used now. Also requires
modification in moodle-performance-comparison to forward the arguments
to jmeter.