Add an optional configuration file

A simple convenience configfile can provide a mapping between VPP
interface names, Linux Control Plane interface names, and descriptions.
An example:

```
interfaces:
  "TenGigabitEthernet6/0/0":
    description: "Infra: xsw0.chrma0:2"
    lcp: "xe1-0"
  "TenGigabitEthernet6/0/0.3102":
    description: "Infra: QinQ to Solnet for Daedalean"
    lcp: "xe1-0.3102"
  "TenGigabitEthernet6/0/0.310211":
    description: "Cust: Daedalean IP Transit"
    lcp: "xe1-0.3102.11"
```

This configuration file is completely optional. If the `-c` flag is
empty, or it's set but the file does not exist, the Agent will simply
enumerate all interfaces, and set the `ifAlias` OID to the same value
as the `ifName`. However, if the config file is read, it will change
the behavior as follows:

*  Any `tapNN` interface names from VPP will be matched to their PHY by
   looking up their Linux Control Plane interface. The `ifName` field
   will be rewritten to the _LIP_ `host-if`. For example, `tap3` above
   will become `xe1-0` while `tap3.310211` will become `xe1-0.3102.11`.
*  The `ifAlias` OID for a PHY will be set to the `description` field.
*  The `ifAlias` OID for a TAP will be set to the string `LCP: `
   followed by its PHY `ifName`. For example, `xe1-0.3102.11` will
    become `LCP TenGigabitEthernet6/0/0.310211 (tap9)`

agentx/network.py

@@ -97,7 +97,7 @@ class Network():
         self.socket.send(pdu.encode())
 
     def recv_pdu(self):
-        buf = self.socket.recv(8192)
+        buf = self.socket.recv(100000)
         if not buf: return None
         pdu = PDU()
         pdu.decode(buf)