Fix interface IP address for DVR with gateway

when an interface is added to a router with gateway, the method
internal_network_added(..) incorrectly configures the IP address
of the new snat interface same as the IP address of the new
subnet interface.

Change-Id: I00d4ff1105a443b89b28359249b8e83fb4c1d157
Closes-bug: #1355087
Merge "Fix interface IP address for DVR with gateway"