improvement: add flag for agentx debugging

agentx/network.py always turned on debugging. It can be useful to have
debugging logs of the main application without the agentx debug logs, as
they are quite noisy.

Now, ./vpp-snmp-agent.py -d will turn on application debugging but NOT
agentx debugging. ./vpp-snmp-agent.py -d -dd will turn on both.

NOTE: ./vpp-snmp-agent.py -dd will do nothing, because the '-d' flag
determines the global logging level.

vpp-snmp-agent.py

@@ -388,10 +388,16 @@ def main():
     parser.add_argument(
         "-d", dest="debug", action="store_true", help="""Enable debug, default False"""
     )
+    parser.add_argument(
+        "-dd",
+        dest="debug_agent",
+        action="store_true",
+        help="""Enable agentx debug, default False""",
+    )
 
     args = parser.parse_args()
     if args.debug:
-        print("Arguments:", args)
+        print(f"Arguments: {args}")
 
     agentx.setup_logging(debug=args.debug)