Add `GANTRY_UPDATE_NUM_WORKERS` and other fixes #40

Job	Run time
shellcheck	7s
Run tests (gantry_cleanup_images_spec.sh)	2m 12s
Run tests (gantry_common_options_spec.sh)	1m 47s
Run tests (gantry_filters_spec.sh)	1m 10s
Run tests (gantry_login_spec.sh)	1m 54s
Run tests (gantry_manifest_spec.sh)	1m 35s
Run tests (gantry_notify_spec.sh)	1m 28s
Run tests (gantry_parallel_spec.sh)	2m 16s
Run tests (gantry_rollback_spec.sh)	1m 2s
Run tests (gantry_service_multiple_spec.sh)	1m 34s
Run tests (gantry_service_no_running_tasks_spec.sh)	1m 25s
Run tests (gantry_service_single_spec.sh)	1m 21s
Run tests (gantry_update_options_spec.sh)	1m 31s
	19m 22s

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