Add "generate resmoke sub-suites" in etc/evergreen.yml

In order to dynamically generate tasks for sub-suites, evergreen needs a function, "generate resmoke sub-suites" that calls "generate_resmoke_suite.py" (which should be changed to generate_resmoke_sub_suites.py) with the appropriate arguments and then calls generate.tasks on the file that was created.

This new function should be usable by any current task that is running resmoke tests.