dws2jgf: check for rabbit.mapping in config

Problem: dws2jgf expects a path to the rabbitmapping file,
but that path is now expected to be found in the config.

Make passing the path on the command line optional, and fall back
to the config if not provided.

src/cmd/flux-dws2jgf.py

@@ -226,6 +226,7 @@ def main():
     parser.add_argument(
         "rabbitmapping",
         metavar="FILE",
+        nargs="?",
         help=(
             "Path to JSON object giving rabbit layout and capacity, as generated "
             "e.g. by the 'flux rabbitmapping' script"
@@ -257,6 +258,12 @@ def main():
                 f"error message was {proc.stderr}"
             )
         input_r = json.loads(proc.stdout)
+    if args.rabbitmapping is None:
+        args.rabbitmapping = flux.Flux().conf_get("rabbit.mapping")
+    if args.rabbitmapping is None:
+        sys.exit(
+            "Could not fetch rabbit.mapping from config, Flux may be misconfigured"
+        )
     with open(args.rabbitmapping, "r", encoding="utf8") as rabbitmap_fd:
         rabbit_mapping = json.load(rabbitmap_fd)
     r_hostlist = Hostlist(input_r["execution"]["nodelist"])