Restore MP-VPN tap functionality

[bmah888]

The "Summer 2015" edition of the MP-VPN (what was used in the iDiscovery tutorial) included the ability to tap into a VPN. This is an important component of Network Function Virtualization. We should consider restoring this functionality. The implementation would be very similar to the iDiscovery implementation, where unicast flows are diverted from the Corsa into the local OVS switch, which then tees off a copy of each packet to the tapping host. Flows are then returned the Corsa for normal packet processing.