Add a CLI output emitter (either to stdout or to filename), based on plan --output/-o flag

vppcfg

@@ -50,6 +50,8 @@ def main():
     if args.quiet:
         level = logging.WARNING
     logging.basicConfig(format='[%(levelname)-8s] %(name)s.%(funcName)s: %(message)s', level=level)
+    if not args.command:
+        args.command="check" ## Default check-only
 
     try:
         with open(args.config, "r") as f:
@@ -106,6 +108,8 @@ def main():
         failed = True
         logging.warning("Planning sync failure, continuing due to --force")
 
+    r.write(args.outfile, ok=not failed)
+
     if failed:
         logging.error("Planning failed")
         sys.exit(-40)