osbuild add new -q, --quiet option

Current osbuild will always print some non output even
when run with `--monitor=JSONSeqMonitor` because of the
unconditional `print/sys.stdout.write()` in `main_cli.py`.

This commit adds a new `-q` option to silence this so that something
like osbuild-composer can run `osbuild -q --monitor=JSONSeqMonitor`
to get pure json-seq output during the build.

The use-case is to run `osbuild --monitor-fd` from e.g. bib and
osbuild-composer so that we get pure json from the monitor-fd
and anything that goes on std{out,err} can be logged as it is
most likely error output.

osbuild/main_cli.py

@@ -103,6 +103,8 @@ def parse_arguments(sys_argv: List[str]) -> argparse.Namespace:
     # nargs='?' const='*' means `--break` is equivalent to `--break=*`
     parser.add_argument("--break", dest='debug_break', type=str, nargs='?', const='*',
                         help="open debug shell when executing stage. Accepts stage name or id or * (for all)")
+    parser.add_argument("--quiet", "-q", action="store_true",
+                        help="suppress normal output")
 
     return parser.parse_args(sys_argv[1:])
 
@@ -162,7 +164,7 @@ def osbuild_cli() -> int:
 
     monitor_name = args.monitor
     if not monitor_name:
-        monitor_name = "NullMonitor" if args.json else "LogMonitor"
+        monitor_name = "NullMonitor" if (args.json or args.quiet) else "LogMonitor"
 
     try:
         with ObjectStore(args.store) as object_store:
@@ -201,7 +203,7 @@ def osbuild_cli() -> int:
                 r = fmt.output(manifest, r, object_store)
                 json.dump(r, sys.stdout)
                 sys.stdout.write("\n")
-            else:
+            elif not args.quiet:
                 if r["success"]:
                     for name, pl in manifest.pipelines.items():
                         print(f"{name + ':': <10}\t{pl.id}")