Document `GANTRY_MANIFEST_NUM_WORKERS`. Add tests #70

Job	Run time
shellcheck	12s
Run tests (gantry_cleanup_images_spec.sh)	2m 4s
Run tests (gantry_common_options_spec.sh)	1m 29s
Run tests (gantry_filters_spec.sh)	1m 11s
Run tests (gantry_login_spec.sh)	1m 51s
Run tests (gantry_manifest_spec.sh)	1m 53s
Run tests (gantry_notify_spec.sh)	1m 32s
Run tests (gantry_parallel_spec.sh)	2m 2s
Run tests (gantry_rollback_spec.sh)	1m 0s
Run tests (gantry_service_multiple_spec.sh)	1m 38s
Run tests (gantry_service_no_running_tasks_spec.sh)	1m 41s
Run tests (gantry_service_single_spec.sh)	1m 27s
Run tests (gantry_update_options_spec.sh)	1m 34s
	19m 34s

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