Allow binding files without IP entry

This is needed for NIC that obtain only IPv6.
Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>

nfdhcpd

@@ -240,8 +240,7 @@ class Client(object):
         self.open_socket()
 
     def is_valid(self):
-        return self.mac is not None and self.ip is not None\
-               and self.hostname is not None
+        return self.mac is not None and self.hostname is not None
 
 
     def open_socket(self):
@@ -606,6 +605,10 @@ class VMNetProxy(object):  # pylint: disable=R0902
                          mac, indev)
             return
 
+        if not binding.ip:
+            logging.info(" - No IP found in binding file.")
+            return
+
         logging.info(" - Generating DHCP response:"
                      " host %s, mac %s, tap %s, indev %s",
                        binding.hostname, mac, binding.tap, indev)