DEVPROD-16911 make number of sub-tasks configurable

[sleaux]

This extends the capability for configuring the number of sub-tasks to all generated task types (previously this was only available for fuzzer tasks). The number of generated subtasks is now configurable using num_tasks, with the default still being to use 5.

Usage:

- <<: *gen_task_template
  name: auth_gen
  tags: ["auth"]
  commands:
  - func: "generate resmoke tasks"
    vars:
      num_tasks: 10

[sleaux]

In addition to the integration testing here, I also tested this in mongodb-mongo-master, splitting some generated tasks into different numbers of subtasks.