Reduce logging on AgentX connections

Previous logging was very noisy when the agent connection to snmpd
drops:

[ERROR   ] agentx.network - run            : Empty PDU, connection closed!
[INFO    ] agentx.network - disconnect     : Disconnecting from localhost:705
[ERROR   ] agentx.agent - run            : An exception occurred: Empty PDU, disconnecting
[ERROR   ] agentx.agent - run            : Reconnecting
[INFO    ] agentx.agent - run            : Opening AgentX connection
[INFO    ] agentx.network - connect        : Connecting to localhost:705
[ERROR   ] agentx.network - connect        : Failed to connect to localhost:705
[ERROR   ] agentx.agent - run            : An exception occurred: Not connected
[ERROR   ] agentx.agent - run            : Reconnecting
[INFO    ] agentx.agent - run            : Opening AgentX connection
[INFO    ] agentx.network - connect        : Connecting to localhost:705
[ERROR   ] agentx.network - connect        : Failed to connect to localhost:705
[ERROR   ] agentx.agent - run            : An exception occurred: Not connected
[ERROR   ] agentx.agent - run            : Reconnecting

Also, reconnects were attempted every 0.1s, but field research shows
that snmpd, if it restarts, takes ~3-5 seconds to come back (note: this
is also due to a systemd delay in restarting it upon failures).
Hammering the connection is not useful.

This change refactors the logging, to avoid redundant messages:
- sleep 1s between attempts (reducing the loop by 10x)
- Either print 'Connected to' or 'Failed to connect to', not both.
- Remove the 'reconnecting' superfluous message

agentx/agent.py

@@ -71,10 +71,9 @@ class Agent(object):
             try:
                 self._net.run()
             except Exception as e:
-                self.logger.error("An exception occurred: %s" % e)
-                self.logger.error("Reconnecting")
+                self.logger.error("Disconnecting due to exception: %s" % e)
                 self._net.disconnect()
-                time.sleep(0.1)
+                time.sleep(1)
 
     def stop(self):
         self.logger.debug("Stopping")