Fix for bug 678

Ensure that when updating a router's external gateway information,
the previous gw IP address is removed.

(cherry picked from commit b988ba05d2a2017a28f7f83c52225d25e3ec71e8)

Change-Id: I3f9c0c41fbb751769339db13153ad673c6a47026
Signed-off-by: Ryan Moats <rmoats@us.ibm.com>