This way we lock the type (proto) of each peer at creation time.
Otherwise, having the possibility of changing the protocol of the peer
makes all the code too complex and there's no good way to prevent the
protocol from being changed at any time creating unexpected issues.
If the user wants to change the protocol, she can do so by removing the
peer and re-adding it through the VTY with the desired protocol.
Change-Id: I47756dddd8f9b8450ba14c914614fd2391d5486e