Fix interface add for dvr with gateway

when an interface is added after router gateway set, external
connectivity using snat fails. Instead of just adding the snat port for
the new subnet, method internal_network_added(..) incorrectly re-adds
all the snat ports with wrong cidr.

Change-Id: I7bfe266288670fba0c90990bf350f43ef7829bad
Closes-bug: #1355087
Merge "Fix interface add for dvr with gateway"