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/network.py

@@ -27,11 +27,11 @@ class NetworkError(Exception):
 
 
 class Network:
-    def __init__(self, server_address="/var/agentx/master"):
+    def __init__(self, server_address="/var/agentx/master", debug=False):
 
         self.session_id = 0
         self.transaction_id = 0
-        self.debug = 1
+        self.debug = debug
         # Data Related Variables
         self.data = {}
         self.data_idx = []