Formalize required argument (#3768)

`--addresses` is de facto expected to be specified, and breaks if not
specified. Formalize that expectation.

# Changed Behaviour

No changed behaviour.

## Type of change

- Code maintenance/cleanup

Author: khk-globus

parsl/executors/high_throughput/process_worker_pool.py

@@ -828,7 +828,7 @@ def strategyorlist(s: str):
     parser.add_argument(
         "-a",
         "--addresses",
-        default='',
+        required=True,
         help="Comma separated list of addresses at which the interchange could be reached",
     )
     parser.add_argument(

parsl/tests/unit/executors/high_throughput/test_process_worker_pool.py

@@ -17,6 +17,7 @@
     "--cpu-affinity",
     "--result_port",
     "--task_port",
+    "--addresses",
 )
 
 
@@ -62,6 +63,7 @@ def test_arg_parser_validates_cpu_affinity(valid, val):
     reqd_args.extend(("--cert_dir", "/some/path"))
     reqd_args.extend(("--result_port", "123"))
     reqd_args.extend(("--task_port", "123"))
+    reqd_args.extend(("--addresses", "asdf"))
     reqd_args.extend(("--cpu-affinity", val))
 
     p = process_worker_pool.get_arg_parser()