Add `GANTRY_UPDATE_NUM_WORKERS` and other fixes #42

Job	Run time
shellcheck	7s
Run tests (gantry_cleanup_images_spec.sh)	2m 0s
Run tests (gantry_common_options_spec.sh)	1m 27s
Run tests (gantry_filters_spec.sh)	1m 9s
Run tests (gantry_login_spec.sh)	2m 4s
Run tests (gantry_manifest_spec.sh)	1m 39s
Run tests (gantry_notify_spec.sh)	1m 22s
Run tests (gantry_parallel_spec.sh)	2m 14s
Run tests (gantry_rollback_spec.sh)	1m 12s
Run tests (gantry_service_multiple_spec.sh)	1m 41s
Run tests (gantry_service_no_running_tasks_spec.sh)	1m 28s
Run tests (gantry_service_single_spec.sh)	1m 22s
Run tests (gantry_update_options_spec.sh)	1m 30s
	19m 15s

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