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.

agentx/agent.py

@@ -28,7 +28,11 @@ class Agent(object):
         self._lastupdate = 0
         self._update_period = period  # Seconds
 
-        self._net = Network(server_address=server_address)
+        try:
+            debug = args.debug_agent
+        except:
+            debug = False
+        self._net = Network(server_address=server_address, debug=debug)
 
         self._oid_list = []
         self._args = args