Fix bug 665 - When updating a router with external gateway information, assign new GW.

original-Signed-off-by: Liran Schour <lirans@il.ibm.com>
(cherry picked from commit 2c792d77235d490d226a157be081e48b65e4b261)

Change-Id: Ibb12b020a1540acad4d5fc2ddeee6039840dcddc
Signed-off-by: Ryan Moats <rmoats@us.ibm.com>