MDL-41913 tool_generator: Refactoring make_form to make_course_form

Also:
- Added a soft password for the
newly generated users so we can use
them to log in the site when running
performance tests.
- Added a method to see the courses-users
relation from outside course_backend class.
MDL-41913 tool_generator: Import .jmx template

Ideally we should generate all the XML based on
modular components so we can also switch templates
between different sizes, this is a first step and
more work should be done in future in this direction.
MDL-41913 tool_generator: Adding a test plan generator to feed JMeter

Also changing course_backend.php coding style
according to codechecker.
MDL-41913 tool_generator: Adding a CLI tool to generate the test plan