Generate resmoke config YAML for a sub-suite

When dividing a suite into sub-suites, the 'generate_resmoke_suite.py' file needs to create a resmoke yaml configuration file for each sub-suite that is being created. To generate this file, the original suite yaml should be read, the selector section should be updated to include the chosen test files for the sub-suite, and the new configuration should be written to disk.