allow an IP to be Null

if the device is in DHCP mode it won't have an IP address therefore the user
must be able to leave the field empty.

devices/models/device.py

@@ -93,7 +93,7 @@ class ConnectedDevice(Device):
 class DeviceInNet(models.Model):
     device = models.ForeignKey(ConnectedDevice, on_delete=models.CASCADE)
     net = models.ForeignKey(Net, on_delete=models.CASCADE)
-    ip = models.GenericIPAddressField(verbose_name="IP")
+    ip = models.GenericIPAddressField(verbose_name="IP", blank=True, null=True)
     nic = models.CharField(max_length=50, blank=True, verbose_name="NIC")
     mac_address = models.CharField(max_length=50,
                                    blank=True,